I’d have as many platforms as space would allow; to make them look fancy, put a 2×4 base down with plywood on top, then a layer of nice oak or ash plywood and rubber stall mats.
Each platform would have a nice bar- nothing crazy- and 10, 25, 45 pound bumpers (or 5, 10, 20 kg bumpers) and 5 kg/ 10 pound metal plates, and 2.5kg/ 5 pound metal plates. Spring collars for all these bars (www.werksanusa.com has the 285 pound HS Set). ideally, a women’s bar for each platform, too.
Squat stands on each platform.
one or two power racks, more if you can afford it. Elitefts 2×2 racks are perfect for the HS setting, get no bells or whistles on them. HS kids don’t need bells or whistles.
adjustable benches, and dumbbells- 5-100 pound set; get pro-style, as HS kids will throw the weights and break them.
pull up bars
Glute Ham benches- these are where I wouldn’t skimp. Sorinex all the way. You need one or two at most.
If you have an unlimited budget, put it into the bars and bumpers first, then power racks, then the squat stands on each platform.