Is it allowed to leave Thailand on an education visa, and what is the acceptable duration for leaving?

no

If the school have holiday, then

It depends on the officer, but here are some suggestions that could help ease any hassle.

I’d probably be hesitant to be out of the country for more than 2 weeks on an ed visa without a family or personal reason. I have gone to immigration & asked, where I was told you can leave for any amount of time. Reality may differ.

As long as you are on break, it should be okay. Bring your class schedule with you to show that you are on a break & not missing classes.

If you miss more than 2 days of class, most schools require a permission form. So bring the approval letter from your school if your stay will include missing classes.

If you’re on a Thai language program, be ready for a pop quiz! They might test your knowledge.

If you want to reenter on the same visa, you need to apply for a reentry permit at immigration. Not sure about length of trip, but I took a 9 day trip to Vietnam while my school was on break and it was fine. But as Graham said, it depends on the immigration officer when you come back in

That is entirely up to the Immigration Officer