Can I extend my education visa and do a border run to Cambodia before it expires?

Nope you cannot get a 30 day extension on an education visa/extension. 😮

AND

if you're going to border bounce, you need to go to the immigration office and cancel the extension BEFORE you go to bounce out

Some land borders won't let you OUT of the country on an "open extension" <- one you didn't cancel before you show up at the border to bounce out and back in free stamp

So I would get documentation from the school saying you stop studying on the 20th of Dec, go to the immigration office a couple days before that, cancel the extension, they should stamp you in until the date you stop studying (the 20th)

and

THEN you can go to the border and bounce out/back to get a 60 day free entry stamp