HCP,
Call facilities management at your school district and ask for someone that contracts out their repair work. Ask that person to recommend several local plumbing contractors. Call 2-4 of those and see if they will come to your place and give you a quote for the work. Try to get three quotes to compare, if you can. Take the lowest quote, or if you like one of them better, you might want to pay a little more for that.
Agree on a price, and list what work you will get for the price. Sign an agreement with them, so both sides are protected. Realize that, if the problem turns out to be a much bigger deal than expected, they will charge you more for that. I'd recommend that you tell them that you want to approve any changes to the cost before they do the work. (Best if that's in the agreement, too.)
Check he the contractor is listed with the Construction Contractor's Board (CCB). You can do this on the net. Check within the CCB site to see if there have been complaints about the contractor. I'm pretty sure they will list the complaint. Not too big a deal if they have one complaint, over a not-so-big a dispute. But, if they have a lot of complaints....stay away, no matter how good their price is.
The contractor will have a $10K performance bond with the CCB. So, if things turn to crap (so to speak) with the contractor, you might have some recourse with the CCB.
Alternatively, you could join Angie's list, and hire someone that they recommend. I've never done that, and don't know anything about them, but it's less of a hassle than what I'd do.
That's all I got.
Go Blazers
I wrote the above before I saw what the problem is. I'd probably get a reference from the SD, check the CCB and have one come over and give a quote. It's not unusual from plumbing companies to have a camera that will reach to your problem, so you might ask them that before you decide on who to hire.