Locating documents and sharing them with others in Teams chats or channels
If you share the form using the Send and Collect Response link, any user that will access that link will not be able to edit it even the creator of the form. I assume that the student are still using the link for editing and collaboration that's why they are able to edit it.
If you want to disable the link to edit the form, you may check on this link and check on the section "Stop sharing a quiz or form":
https://support.microsoft.com/en-us/office/shar...
Thank you