How to Delete A Student Guest Account/Chat

Being an Admin isn't easy. Your problem is a little bit more complicated and bigger than what it seems on the face of it. To provide you with remote assistance is not easy, since there are so many possibilities. You may need to get some professional help.
1. There are policy settings which pertain to your organization, and then to Ms Teams (in your organization) where you can prevent private chats, adding guest accounts and a ton of other things. If you leave all these open, students can even have video calls on Teams without your knowledge. There are policy templates that Microsoft has created that you can apply organization wide - that are suitable for students. You would have to do a bit of reading up on Teams admin. For starters, look at this online document Keeping students safe while using Teams for distance learning.
2. Microsoft has now released a private version of Teams which works with a private account (not school or work related). You would have no control over that - it is like the student can install any other social network app and use that to communicate - you basically need to restrict that kind of thing on the iPad level.
3. There are parental controls on the iPad itself which can be set with what is called by the misnomer 'Screen Time'. That is where you can set all kinds of restrictions.