Hi @infohta,
Welcome to the community 
I believe all the Teachers and Students are stored in the Users collection and you Identify if that is a student or a teacher from a true/false property? ( True for teachers and false for students or opposite)
Then in the Users collection you can create a Many to Many relationship between the Users collection and the Users collection. ( I believe you already did this )
And for selecting multiple things you can use the Multiselect Dropdown component!
You can connect that to the Users collection and filter it by the true/false property to show only students and add Update actions to add and remove like this. : Multi Select.mp4 - Google Drive
But a issue with this component is once you select the students and close the app and come back again the component doesn’t show if this is selected and for all records the component display unselected.
For this you can make a own one. Not like a drop-down but I believe you can do that too! ( Didn’t try that yet )
You need a custom list connected to the Users collection and filterd by the true/false property to show only students and a toggle inside the list that connected to that Logged in user’s> many to many relationship.
This video and post may help here : Multi Select.mp4 - Google Drive , Assign/uers assignment - #17 by dilon_perera
Hope this helps!
Thank you