I was a music teacher with several music schools. I did not have to deal with summer breaks etc. If you generally have over 30 hours per week for lets say 4 years in the same school, you do not have to put in each year separately.
Just get a letter from the school stating you worked " X " number of hours from year " xxxx to xxxx " with a summer break each year and this would do.
If your hours changed from month to month, then it would be better to put in each month/year else there are no problems.
You get maximum points for 3 years of work experience, if have more than that with other jobs, don't bother getting reference letters for those since you wont get any additional points(you should however mention them in the previous history section).