Well theoretically, he could call up a militia (of teachers) ;-0 but I think the more proper way of looking at this is that Trump is looking to legislatively remove barriers to teachers voluntarily exercising their 2nd Amendment rights, i.e. eliminating an infringement on a select group of people. A repeal of the Federal Gun-Free Zone act would be in order but even that wouldn't effect the myriad of state laws that conspire to make it impossible for most teachers to legally carry on the job.
It will be interesting what they come up with. AT LEAST we have a President talking in the right direction, which is more than I can say for most politicians.