Before assuming it needs repair, check whether the lens’s focus limiter switch has been engaged. Many macro lenses, including the Tamron SP 90mm Macro, have a limiter that restricts the focus range so the lens doesn’t travel through the full distance range during close-up work. If that switch is set to the macro/limited range, the lens may seem “stuck” and won’t focus on distant subjects. Inspect the barrel for a focus limit switch and set it to the full-focus range. Since the lens belongs to your school, avoid disassembly or self-repair beyond checking that external switch.