Full miltary outfit and chest rig does not look like a costume. Going as batman or spiderman is a costume.
I hope if you do decide to go miltary you will leave the pistol holster off and no magazines in the pouches.
And even if you clear it with the school which I doubt you will, neighbours in the surrounding house could still call the police. You may want to visit all the houses along your route to school and let them know also.