I'm a junior that's school year will end in mid-June.
P.S. Any suggestions on who should proofread the essay
The answer depends a little on when the programs start, but I'm going to be honest - unless you're going for a top tier 1% school, its not something you probably need to be all that concerned about. I would say personally that I would recommend that for each application, that you consider what they're asking for and how long you think it would reasonably take to get that accomplished, and then give yourself an extra set of time to get the essay looked over (maybe here as you can get it peer reviewed for free and I found my peer reviewed paper quite helpful, or as always you can easily find online college essay reviewers).
If the program application close date was three months away, and they want a one page essay (which is a common length that I've had personally - 1 to 2 pages) then I'd say three months is a good time to dedicate to writing it out a few times and getting it looked over.
However if you're a junior and you wish to go to a top tier school when you graduate, I'd say doing the research over the summer of what the essay requirements would be and getting a head start over summer break would be a good idea! It's always a good idea to have at least a basic format to work with and improve upon but I do not think that for most personal essays that you need to be worrying about them in your Junior year.
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