Can I make a low-cost black-and-white fixer if we run out of hypo?

The fix solution you seek is one of the most inexpensive chemicals of the black and white process. You can use most any fixer formulas. In other words, fixer for X-ray, fixer for film and paper. You might buy some from a local camera shop or photo studio. In any event, the stuff is benign. You be at ease, allowing your students to work with it. It has the smell of vinegar because most formulas contain acetic acid (concentrated vinegar). Call or go to your local hospital, they likely will give you a package of X-ray fixer after you plead your case. You can test - swish a snipping of 35mm tongue in paper cup filled with fixer. Do this in the light. You will see the film change from opaque to transparent. Time this reaction. Fix time for film and paper is double the time it takes for the film to clear. If the film does not clear when performing this test, the solution is contaminated of exhausted.

Hospital and dental have mainly phased out chemical based X-ray for digital however, some chemical X-ray is likely in use. Just as likely, the local hospital will have some boxes of fixer in the basement. Make some calls.