This is the classical catch 22 situation.
On the one hand, we want our best bachurim to learn as much as possible, on the other hand, we want the best bachurim to be the counselors of our children for the influence they can have on them.
Practically speaking, extending the yeshiva ketana school year through Tammuz albeit on a lighter schedule is not all that practical considering counselors are STILL needed for the time that the children would be in english classes. Moreover, kids would still be home at 5pm with nothing to do. The last 3-4 hours of daylight would make everyone crazy with not much to do.
There are a number of fine yeshivos that do NOT require their high school bachurim to attend the “yeshivas camp” for Tammuz and the bachurim are allowed to work in camps. In fact, it does not appear that camps have a lack of quality counselors.