Hello everyone, I wasn't sure which category to put this in, so tell me if it needs moving

Our school's music equipment (like in all schools) sucks ass. Whenever the principal speaks at an assembly, or when a band plays, it sounds so muddy that you can't understand a word they're saying.

We have some Shure sm58 mics, all of which are pretty old, and some ancient Peavey power mixer that looks at least 15 years old. I can't remember what it's called, though

Now, I'm planning to play in a gig, and I'd like the vocals to be comprehensible. Is there any way to make the mic a bit clearer?

We do actually have a few other PA heads, but they're just as muddy and there's nothing you can do. Turning the treble up doesn't help.

Suggestions, anyone?