I'd go about it differently, first I'd approach the head of the school, then I'd go to the people bringing in equipment and let them know what is going on, nobody "owns" the customer in any way shape or form nor can demand anything of them or threaten "their" choice of repair people or shops.
If it ever happens again with you being threatened or harassed, I'd tell them straight up, back off, shut up, or everyone would know about their tactics.
If customers whom own these machines don't know their choices are being eroded behind their backs by the places they either buy their machines from, get parts from, or get repairs done, that's plain not right to those equipment owners.
Anyone I hire, whether its an operator who runs machines, someone I sub work to to help me out on a job get things done, someone in the office or even independent mechanics to come help out, are in essence employed by me, and I want to know if anyone threatens them in any way shape or form while under my hire, or because I hired them,if they are or do get threatened while in my employ, **** will hit the fan and I'll take care of it in no uncertain terms and it will end today.
I've taken things to tech schools to get repaired before, the way I looked at it was a donation of sorts, I want kids/young adults to get experience in doing repairs, no matter what or who they end up working for, I bought the parts for them they needed, then made a donation to the school afterwards to help them buy tools or whatever. Its my way to DONATE and nobody will tell me who or what I can or can't donate to, when or where. If this takes money out of those dealerships of the machines I took to the schools, too bad, its not their money till I determine its "theirs", until it leaves my hands, its mine and I'll decide who, where and when it goes. period.
I'd then ask those who hired you how they feel about a lot of things, starting with do they approve of YOU being threatened while under their employ?? and what they plan on doing about it?? as they say toss the ball back into their court and find out who your working for so to speak. Best of luck