Im sorry i dont remember if your child is verbal or not...
Anyway, Daniel use to throw tantrums alot until they created a picture schedule and stuck to it. Following a picture schedule enabled him to know what to expect and lessend the frustration. Picture or even word schedules work well for verbal or non verbal children.
They may also be reinforcing the tantrum with too much response or by wavering, so hes learned he tantrums he wins. They must be matter of fact and try to ignore the behavior.
Its important I think, to have a behavior plan. Its also required by IDEA for any child whose behavior influences his learning ability. THe plan should inlcude ways to keep behavior from escalating, what to do in case of extreme behavior ect. And should be followed...it also allows those working with your child a better idea of what should be done.A team meeting to discuss it might be a good idea.