How am I texting while my phone is on airplane mode

[AC Question]

Ok so my school has really ****ty service and while im at school i turn on airplane mode to conserve battery but also turn on wifi so i can snapchat facebook chat ect.. My idea of airplane mode is that you cant send or receive text messages but the other day i received a text from my brother while at school and i was really surprised and i thought that i couldnt text back but i tried and it went through to him so if anyone could explain how this is possible that would be very helpful, i have a galaxy s6 edge and my provider is version