What the benefits of summer school when working on masters degree?
The trouble with "what you're supposed to do" is that it often applies to some imaginary archetypal student or, worse, students of a very specific class and privilege. Not all students have the means to go without working, to help pay their own way through school. Academia is just starting to wake up to this fact, thanks to the consciousness raising of folks like Sarah Kendzior.

If you can find a part-job related to your field of study, then it's definitely OK for you to work over summer break. I was able to do this while in my MLS program.

If you are unable to find work related to your field of study--and if you aren't involved in student organizations or showing that you're otherwise productive in your "non-school" hours by starting to attend conferences, write papers, etc--then I could see it being a drawback to have a job in your "off" time.