Does A school count as Active Duty?

I will be going into the nuke program and my husband is out of work so he will need our BAH allowance while I am gone. I hear you start getting BAH as soon as you start active duty? Is this true and is A School considered active duty. Nuke school is about 18 months long which would be a long time to go without recieving BAH if it isnt considered active duty. Please help.