Do not activate the burner near the school district you work for. You will have to give an address and will get a mobile number that is near the area (T-Mobile and Verizon do this). Go a county or two over in a random direction, pick a random address from that area, and use that when you activate the burner.

If you activate the burner too near the school district, that leaks a couple of bits: Whoever made the call (you) is near the school district; whoever blew the whistle works close enough to the school to see what's going on; that narrows down the list of suspects for retaliation, which you do not want.

If you bought an Android burner phone, you'll need a burner Gmail address as well. Again, set that up someplace not particularly close to the school district. It's not essential but if you're going to do the one, you may as well do the other. Additionally, just in case they decide to subpoena the Big G you want whatever they get to not be too close to home.

Don't buy the top-up card at the same time as the burner phone. I don't know how important that is in the final analysis, but it makes sense to spread things out a bit.

When you go to activate the phone and make the call - and this is "holy shit" important - leave your personal and work phones at home. Powered on. Not in airplane mode, you want them associated with the usual cellular tower near your house. If you don't, and the district investigates prior to retaliation, they'll check to see who took their personal and work phones with them on any weekend trips, and whether or not they showed up from where the whistleblowing call was made. That leaks bits of your identity which you don't want. Instead, you want them to think that you've been at home as usual hanging out and not doing anything unusual. So, plug 'em in, leave 'em at home, and go for a drive.

If your car is always-on (i.e., it's relatively modern so it's sending telemetry all the time over cellular), I don't know what to tell you. I hope it's not.

Go on your friendly, happy drive to see the world. Again, drive for one or two counties. Make the call. Drop the dime. Deep the throat. Leaks the docs. Whatever. Then yank the battery out of the phone, wipe your fingerprints off with a piece of cloth (you bought a burner phone to blow the whistle, going a little farther than it seems you need to go is worthwhile), and toss the battery. Pull the SIM card, too. If you bought a crappy flip phone bust it up a little. (You can bust up crappy Android burner phones, too, but it's a little harder because they're solid slabs.) Go somewhere else and toss part of the phone into a busy trash can (at a fast food restaurant or bookstore or something). Go somewhere different and toss a different part of the phone. Go someplace that has a bathroom you can use, use the bathroom for its intended purpose, but before you flush toss in the SIM card (this makes sure that there is more mass that goes down than flushing an unused toilet and ensures that the SIM card goes down).

Again, if you're worried enough that you bought a burner phone, go a little farther than you think you need if you're afraid of retaliation. Go big or go home, right?