Get rid of a single long summer break and the entire concept of a “school year”. Divide the year into quarters. Students can start in summer, spring, winter, or fall.
That's much harder to do than it sounds because now you have 4 groups 3 months apart in the curriculum. The only way to really do it would be to segregate the kids from the different start times into separate classes otherwise it'd be pretty impossible to teach. There will also be a slight variation in class size depending on the quarter it's for. as August and September are over represented as birthdays.
I do think year round schools probably make more sense overall so there's not such a huge gap where kids forget so much.