When, exactly, do the March/April 2nd term school holidays at Bangkok's primary (pratom) schools start and end?

They start when the school decides to start them.

By the way, are you talking about students or teachers? I think most students are already out.

As Scott said, each school pretty much makes up their own schedule. However, most schools finish their second semester somewhere between Mid February and early March and start the next school year sometime between the 1st and 15th of May.

International schools are usually on an entirely different schedule though.

Last for the kids today!! One more week to go for teachers at my school. Start back end of May.

By the way, are you talking about students or teachers? I think most students are already out.

Teachers. Asking, cause almost zero communication between Thai staff and foreign teachers. All at last minute.