One-third of the classrooms have 30 desks, so there are 1/3 * 15 = <<1/3*15=5>>5 classrooms like that.
The rest, which means 15 - 5 = <<15-5=10>>10 classrooms, have 25 desks.
One-third of the classrooms can fit 5 * 30 = <<5*30=150>>150 students.
The rest can fit 10 * 25 = <<10*25=250>>250 students.
So in total, the school can be filled with 150 + 250 = <<150+250=400>>400 students.
#### 400