"back of" is wrong. "back from" seems the most likely correction -- presumably meaning that the person has returned home after being away all summer working? "my last year beach cafe job" is ungrammatical, and the overall sense of being "back" from a job that happened "last year" is not very clear.
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SopranoI would tell the student his sentence is too long and mixes too many things.Good point. Still, it is possible (with the right constructions) for a more advanced student to put even more information in one sentence.