CMOS transistors for educational purposes
As always, Wikipedia ia a good place to start and here's a CMOS circuit and 6 transistors is what you asked for. Of course, it's not the most obvious of circuits. I'd recommend that it's time to transition to CMOS ICs, with the 74HC series my preference. You can make an XOR gate from



Since a 74HC00 package conveniently has 4 NAND gates, it's a good place to start. Get them to build up one XOR, and then show that 4 of those fit in a single 74HC86 package, and you're on your way to teaching them about the benefits of integration, starting with SSI (small scale integration).