Probably it is a bad idea. But life is nothing but choices either good or bad we have to learn live with.

My first concern is the birth control. And what happens if it fails.

There are other good ideas in the thread: to charge a token amount of rent with a rental agreement. You have to have a way of legally remove an unwanted guest.

Probably having some strict rules put in writing for her - even if she is a real angel - would help also in the long run.

Most people won’t spend their life wih their high school sweetheart. This is a very strong thing you have to able to defend both kids from. They need the freedom of the ability to break up without putting any one in an awkward situation. The convenience should not lock the girl into a situation that she could have been better out of. Or the boy would feel guilt so he wouldn’t break up with her, so she won’t turn “homeless”. You can never know.

Maybe getting her a student apartment somewhere else would be a way better option. Maybe there are goverment or student grants for her to apply?

Spell check is one of those.
Writing our own letters. That was actually a skill

Literal translation it is a bit funnier in original:
“If you’re bored, sharpen a stick and stick it up your butt so you resemble a donkey more.”
Basically there is 0 reason to be bored in this life.