I am a teacher so we go in the summer every year. I have never been in June (we don't even get out until June 25th!!!) but have been every week in July and the first two weeks in August.
It is definitely hot and the week of 4th of July was REALLY crowded. However, if you drink, take breaks in the afternoon, and have the attitude that you will see what you see and enjoy yourself, you will be fine!!!
We usually eat lunch at 11:30 am, before the crowds set in at the CS places, which gives us a nice break. We usually leave the park between 1 - 3 and head back to the resort. Sometimes we go back and sometimes we don't.
The biggest thing is drinking lots of water. It helps a lot!!! Utilizing the Fast pass system is a must. We plan our visits to the parks based on when the parks have night time EMH. We go to that park the NEXT DAY--ie: if MK has EMH at night on Wednesday, then we would visit Thursday (we figure that since a lot of people were there late the night before, the crowds might be less). We try to avoid the parks that have Early EMH as well since those tend to be the busiest parks.
Have a great time!!!!