From the rehearsal room to the arena: how 100,000 children practice for 6K UNITED! with cori

Picture a sold-out arena (including venues such as the SAP Arena in Mannheim, the Olympiahalle in Munich and the Barclays Arena in Hamburg), with 6,000 singing primary-school children inside, accompanied by an 18-piece band. That is 6K UNITED!: a project founded by Prof. Fabian Sennholz that brings more than 100,000 children from over 1,500 schools and choirs all across Germany onto these stages every year.

Since May 2026, all the children in the 6K choirs have been able to practice for the big concerts with a practice space in the cori app. It is free and built specifically for children aged 7 to 14. This post is about what is actually inside that practice space and how the partnership between cori and 6K UNITED! came about.

But first: the problem.

A 6K choir has, like almost every school choir, one hour a week. In that hour the children learn the largest part of a song. Then they go home, and for six days the song doesn't live anywhere they can reliably find it again: a YouTube link that gets buried in the parents' WhatsApp group, a printed lyrics sheet that gets crumpled in a school bag, a "we'll come back to that next week" that quietly turns into: "let's start the song again from the beginning."

What a 6K year looks like in practice

A bit of context before we get to the app. Schools and choirs sign up with 6K, receive a songbook, choreography videos and access to teacher workshops, and then work through the school year on the same repertoire as all the other 6K choirs in their region. In May or June everyone meets in the arena (Munich, Düsseldorf, Hamburg, Berlin, Mannheim, Hanover, Vienna) and sings a concert on stage together with the 6K band.

The repertoire is a mixture that surprises you the first time you hear it: current German pop (for example Lotte's Mehr davon, Blumen und Beton from the Schule der magischen Tiere soundtrack), folk songs such as Du, du liegst mir im Herzen and, again and again, a classical line. The Ode to Joy comes up regularly. The whole thing is designed so that it works even when the teacher leading the choir has no musical training.

How the partnership came about

The videos and songbooks already existed. What was missing was a place where a nine-year-old can find them again on a Sunday afternoon, and a reason for him to come back on Sunday evening. Practicing at home has the same underlying problem as homework: without a teacher in the room, the external nudge is missing. We wanted to build in a small, visible bit of progress that says: "You've already practiced, keep going tomorrow." Something that keeps the arena concert alive in the children's minds without anyone having to nag.

A few meetings turned into a prototype, and since May 2026 there has been a 6K area in cori that is included in every participating choir's registration fee. 6K funds the development of this area through the partnership, and for participating choirs it stays free.

What is actually inside

Children get an access code that takes them straight into the 6K area. Three functions are worth describing in a bit more detail.

Learning videos, with live lyrics and jump markers

In cori every 6K song gets its own practice screen: the choreography and voice-part video, the lyrics running along live and highlighted at the spot currently being sung, and jump markers to the sections that are actually referred to in lessons (warm-up, left-hand choreography, right-hand choreography, verse, chorus, bridge). "Practice the chorus of song 3 tonight" works as homework because of this: the child taps Chorus, lands exactly there, the lyrics run along, and they can sing along straight away, without pressing pause and rewinding.

That sounds like a small thing. In conversations with teachers, it is the function that gets mentioned first.

Interactive exercises

For some songs in the 6K repertoire we built interactive exercises. The child sings along, and the app shows straight back whether the pitch is right and the rhythm is solid. No rating score, no grade: a visible trace on which the child can see for themselves where they are right now and where they still need to get to. The analysis runs entirely on the device, the microphone signal never leaves the tablet or phone.

An interface for children

The functions and the interface of the normal cori app have been adapted for children in the 6K area. A child who opens the app sees a list of songs, a video and a streak counter. No member list, no audio mixer, no settings menus, no "tap here for more options".

If a child first has to ask a parent how the app works, they won't use it on their own. And on their own is exactly the moment when practicing has to happen.

In working with 6K UNITED! and in designing the children's area, these points mattered to us:

• No voice recording for children. Where sing-along functions for children exist, the processing runs on the device, and the audio data never leaves the tablet or phone.
• No personal-data features. No chat, no public profiles, no shared playlists between children. The 6K area is a closed practice space inside the cori app with one clear purpose.
• No advertising SDKs, no third-party analytics with personal data. There are no advertising networks in the app. The 6K area is free because the partnership funds it. Not because behavioral data is being sold in the background.
• No accounts or email addresses for children. The child enters the code and practices. Parents install the app on the family device; nothing more is needed from either side.

All data is stored on German infrastructure. These standards apply not only in the 6K area, by the way. The regular cori app too, whether used as a choir app, a digital rehearsal room or a practice platform, processes data in a GDPR-compliant way on servers in the EU, without advertising networks and without selling behavioral data. In addition, the 6K area does without everything that works perfectly well for adult choirs, such as voice recordings, member lists and social features.

How to get started

If you are a teacher or choir leader and already have a 6K access code in your inbox:

1. Download cori.

   

2. Enter the 6K access code the first time you start the app.

3. Pass the same code on to the parents of the children in the choir. They install the app on the family device the child otherwise uses for schoolwork, enter the code and land in the 6K area.

There is no account-per-child setup and no collection of email addresses, because there is nothing to collect: the code is the only thing that connects a device to the content of a 6K choir.

We answer detailed questions about devices, data protection and upcoming features on the page 6K UNITED! in cori.

What changes in rehearsal

I'm careful with big promises about what an app changes in rehearsal. What teachers from the pilot tell us, in essence: the hour doesn't get any longer, but more of it goes into the actual music instead of catching up on what has been forgotten since last week. A Friday warm-up shifts from "let's remember the second verse" to "now let's work on the dynamics."

A short note for choir leaders outside 6K

The 6K project is the biggest deployment of cori so far, but the underlying observation, that the gap between rehearsals is where progress is won or lost, applies to almost every choir we talk to. If you lead a regular youth or adult ensemble and are thinking about exactly this problem, the cori feature overview is a good place to start. The same building blocks, the same GDPR-compliant data processing on European servers, just without the child-specific trappings.

Questions about the partnership, about setting things up in your choir or about cori in general can go to info@getcori.app. We usually reply within one working day.

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More about the partnership: 6K UNITED! in cori | About 6K UNITED!