I've still always liked the idea of them making one with the SPLAT team as adults having to defeat the headmaster one last time, but its never likely to happen sadly. I don't think most of them are even acting any more for a start.
That needn't matter.
I think off the top of my head only two of the six SPLAT team are still acting, three were literally one show only (this one) and the other did something else and then jacked it in.
We can just blame puberty for the new heads
Of course the second and third series the main character wasn't a Headmaster at all. The second series he was a director and the third he put himself in charge of the Hyperbrain project. And of course he wasn't officially dead at the end of the third series either...