Hey there, Sophia Volz
When looking at their support article, it says that you can use your Microsoft account to verify your age by signing in with your Microsoft account. Before doing this, please can you sign into your Microsoft account overview page here: https://account.microsoft.com/profile
Please check that your date of birth is correct on the Microsoft account. If it is, please navigate back to your GrupeMe app and see if you have an option to sign into your account with a Microsoft account. In the article, it does mention you can only have one Microsoft account connected to your account at a time. You mentioned you have a work/school account on Microsoft Entra ID. I am thinking this doesn't count as a Microsoft account due to it being a work/school account.
You can find out more about this in this article here: https://support.microsoft.com/en-US/accounts-billing/manage/can-i-sign-in-to-groupme-with-my-microsoft-account
You can also find out more about how to confirm your date of birth in this support article here: https://support.microsoft.com/en-us/groupme/why-is-groupme-asking-for-my-birthday
It is linked to both my phone number and my college email address, and I can verify ownership in any way necessary. My account is tied to my college Microsoft account, clearly demonstrating I am above the age limit.
When you look at Microsoft work/school accounts, it doesn't allow you to enter the date of birth for the account on there. The only thing IT admins can do is add if the account is a child or adult account.

When you view the available options, you have four to choose from.

So depending on if this has been selected (Because it isn't compulsory you select an option here), will depend on the classification of the account. The chances are, because the account doesn't actually hold a date of birth, it probably won't be used as part of its verification.
In addition to this, many organisations will just have groups that hold staff accounts and student accounts and not complete this section for the account.
I'm simply asking for the opportunity to verify my identity and update my account so I can continue using it. I hope there is a way to resolve this without permanently deleting a long-standing account.
If this doesn't work, please can you reach out to their support team via this link: https://support.microsoft.com/en-us/groupme/how-do-i-contact-groupme-customer-services
Their support team is better equipped to tell you what your available options are for you.
Hope this information helps,
Nathan