Over the school year, many students have a rigid and full schedule that they follow, with little free time. During the summer, however, students suddenly have a huge amount of free time. This is a good time to capitalize on your interests and passions, and boost your college application. Don't let free time go to waste!
While you may be enticed to spend your summer doing little more than watching Netflix and hanging out with friends, remember that other students will be spending their time trying to outclass you. Summer, don't get me wrong, is a great time to relax and recuperate from the previous school year. But you should also attempt to spiffy up your resume and experience levels with something a little extra.
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A great way to help your application is to participate in activities that show what you are passionate in. Are you really interested in politics? Volunteer (or intern) at your local congressman's office, attend Boys/Girls State, and/or participate in a campaign. Are you into science? Try to get an internship with the NIH, search out a local research opportunity, or volunteer at a hospital. There are numerous opportunities that can be taken advantage of; it is just up to you to find them. Try planning out your summer far in advance, as some summer programs/internships require an application due in early spring. If cost is an issue, try getting a job. Colleges love to see people who work hard and have experience in various atmospheres.
Volunteering is another great way to spend your summers. With flexible hours, it is often more forgiving on your schedule. Additionally, it is usually easier to get a volunteering gig than it is to get a job. Showing to colleges that you are more than just a straight A's and perfect SATs robot is very important! They want to admit people, not test scores.
Another good option to spend part of your summer doing is studying for standardized tests. During the school year it can get tough to fit in practicing, so summer offers a great time to familiarize yourselves with the test and be ready to take it soon after the next school year starts. Try buying one of the official study guides and work through the tests they give you throughout the summer; it'll help more than you know!
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Some universities, such as Stanford, Harvard, etc. have summer immersion programs for students to take. Some may be in a language, some may be in the sciences, and some may be in other subjects. These programs (which usually cost a hefty sum) can help you get a feel for the college while at the same time portraying you are deeply involved in your interests. Of course, if these cost way too much, there is nothing wrong about learning something new over Coursera or EdX.
Overall, make sure to do your research. Start around the end of first semester thinking about what you might want to do. Nothing is worse than looking for things to do too late (which is what happened to me!) and not being able to find anything impressive. You will notice that many of the people who get into Harvard, Yale, Stanford, etc. spent their summers doing VERY impressive things; often more impressive than what some adults do. That said, I got into great colleges with my summers spent working at Subway, volunteering at a local clinic, and swimming competitively. None of those are overly prestigious or whatnot, but I was able to twist my application to portray how these activities best fit into my summer.
For Summers Before Senior Year: Get started on those college admissions essays! I was able to get about half of my essays done before school started, making for a much easier college admissions process. Most colleges keep prompts the same/similar each year. Additionally, the Common App usually releases their prompts mid-summer for students to start working on. Don't procrastinate!