It’s still summer, don’t worry! (Believe me, I really need my summer break.) But for your middle-grade characters, school is probably an important aspect of their lives. Thus, you’ll need to develop your in-class time well.
Start with the basics: what grade is your main character in? There will be technical details to take care of based on this, things like how long recess is, or whether the students are allowed off school property at lunch. Grade also tends to determine what the children do at recess: for example, younger kids might play wallball and hopscotch, while older, nearing-teen students would rather hang out on the pavement and talk, or play volleyball. How about who packs lunch? Maybe grade 4s still get their PB&Js (with crusts off, of course) wrapped by Mum, but grade 7s need to make their own sandwiches and warm up the Thermos.
Once you have little details down, think about the education aspect of school. Will what your characters learn be important in the rest of the story? Try finding out what a typical grade 6 curriculum looks like; workbooks like SummerSmart 5-6, usually intended to help students keep expanding their knowledge over the summer, would be a good start. (Though if you have kids, please don’t give them these workbooks. Trust me — I speak from experience — they contain mind-numbing exercises which will not expand their knowledge. They’ll give you insight to the curriculum, but that’s about it.)
Often, school is a part of your protag’s growing up, but may not be directly related to the plot. In that case, how school life influences and changes his or her perspective and outlook will be the most important thing to focus on.
And you probably want to give the stereotype "all kids hate school" a rethinking. I happen to have a younger sister who's currently lamenting the fact she can't go to school and see her friends daily. Can you believe it? ;)