How is the link for the Meet being created? We have found that if they are Calendar events, kids can still get in first (They can also use those links later and get in without a teacher. Calendar Meet links don't expire until 90 days of nonuse. Savvy kids!) We are also an Enterprise district. 

Events made with nicknames and in Classroom seem to do a better job keeping the kids out. I haven't had anyone say kids are getting in early with these links, but I'm sure there is always a possibility. I'll have to check out the "brute force" method mentioned to see if it works for us, but it's not something I've heard yet in our district.