Teen!John and Sherlock are long-time boyfriends, even though Sherlock hasn't presented yet. John has already presented as alpha and has spent the time since he presented taking care of Sherlock and generally keeping people from bullying him in school. They know Sherlock will present as an omega (maybe there is some prior dna tests that families can run on their kids in this AU?) and are just waiting, taking things slow, NOT HAVING SEX. Just being good kids. One day, while at home with John on his way over, Sherlock feels a little sick. John gets there and takes care of him, they don't know that it is actually Sherlock going into heat until its too late. They have sex for the first time, but they use condoms and they don't bond because they know they are too young. Everything is rainbows and sunshine.

A month or two later, Sherlock finds out he is pregnant. His family is very old fashioned and wants him to put the baby up for adoption because they think he is too young to take care of it, but neither he or John want to do that. They want to have their baby and become a family. Sherlock's family says that since Sherlock doesn't have an alpha (because he and John aren't bonded) they can decide the fate of Sherlock's baby.

Bonus points for John being a bamf and standing up to Sherlock's family

Extra bonus points for both boys having their own doubts but ultimately wanting to be together and make the other happy