It's typically been tempered glass at the rinks I've been to. Tempered glass is thicker, at least a half inch. Lexan types are thinner, by quite a bit. The sound signature is quite different too. Glass is a higher pitch, but pretty dead. Lexan is a very solid WHACK when the puck hits it. The one Lexan rink (that I can't remember the name of right now) had boards and 'glass' that wasn't very new.
With the tempered glass, it's very tough, but they all have a sweet spot (for lack of a better term) that will make them shatter. I've seen teens crack and adults break glass with shots that honestly weren't very hard. It's just a matter of getting hit in the right way in the right place.