Young mum

I'm 15 and pregnant with a little girl, my boyfriend is 17 and we have nearly been together for 2 years and we are happy!! I'm still in school and plan to leave when I'm 36 weeks but I'm currently 33 weeks. And it's hard because I feel so out of place with everyone I have to wear leggings because I can't fit into trousers and every other girl at my school wears a skirt I can't even wear the school top I have to wear my mums work top! I feel so out of place I can't wait to leave. I got into collage for after the baby's born and Iv left school but my sister called me selfish for going to collage and leaving my baby with a child minder! I don't want to leave my baby but I feel it's the best thing for me to carry on with education and to give my baby a better life then a lazy ass mum! Am I being to selfish?