In general, wedges are more dressy than sneakers or sandals, but they can be dressed down with casual outfits. For example, wedges can be paired with jeans and a T-shirt for a more relaxed look, or with a skirt and blouse for a more formal occasion.
If you are unsure of whether wedges are appropriate for middle school, it is always best to err on the side of caution and choose a more conservative shoe option.