No, physiologically speaking, something different happens in middle school track runners and they get much slower if they do tempo or lt work.
Really, middle school track runners should NEVER run more than three miles a week at any age. They will get much better by doing wind sprints and walking than anything else. Show this to your coach if he/she is doing fancy stuff like "tempo runs." Wind sprints are the way to go.
Always, always, no matter what kind of coaching you or your kid gets, question it. Question it on here, and show the coach all the responses you get. Do a mix of the responses instead. Make sure the coach does what you say because your coach can't be right; everyone else is always right.