Middle school isn't as bad as all the other definitions posted. In fact, I had a wonderful time in middle school. It's generally from grades 6-8 or 7-8, but there are some other variations on that. Most of the students are going through puberty at this time, which may make them feel uncomfortable. You get to meet a lot of new friends and take some good classes, though you'll end up discarding a lot of the information you learn.
"I went to ***** Intermediate in Sunnyvale, CA."
"That kid's kinda short, but he has a little bit of facial hair. He must be in middle school."
"Meiosis? Mitosis? When will we need this in real life?"
Answer: Middle school.