What you have described is a typical pipe bomb and that is why is might have exploded.  Many years ago when I was a fledgling un-eductaed aspiring chemist I was quite interested in rocket and fuel and the like.  I remember one particular "test" where we used a paper towel roll tube and fashioned a nozzle out of clay.  Upon ignition the clay nozzle was blown out and the fuel proceeded to burn at an accelerated rate and completely consumed in just seconds.  Yes we did have face shields but were quite distant from the "Rocket Motor"!!!!  What did I learn?  Don't fool with this stuff.  We then proceeded to spend $ on Estes rockets and then enjoyed the hobby!!!!

My surprise here is why did this team not realize that they were Making a "Bomb" in the first place and as reported that this group was sanctioned by the school- there was an obvious lack of "understanding of the principles" and "supervision"??????

Mike