I'd suggest using a small toroid core and wind yourself a little 1::3 transformer, and drive that with a 3.3v square wave. At 25kHz, you wouldn't need a beefy core. Of course, I'm assuming that you don't actually require a square wave across the piezo element. If you do, then I'd follow zeke's suggestion to build a small voltage doubler, then you can chop that with a BJT switch to get a pretty reasonable square wave.
A quick web search (duck duck go: at command set +class "HC-05") turned up a bit of docs (on slideshare) that mentions it. From what I read (not the whole thing) it looks like many of the parameters are in hex for many of the commands. The next entry in the document, for example shows that for "AT+IAC=<PARAM>" that the default value is 9e8b33. So I'd try converting 99 to hex (63) and sending: AT+CLASS=63 (Note: I'd've put in a link to the documentation, but as a new member here, I didn't want to run the risk of being marked a spambot due to having an offsite URL in a short