To the person starting their senior year,
Senior year flies by, so take your time. Don't rush into it and wish for it to be gone. Sit back and enjoy yourself. Take notice of things. One of my biggest regrets was not spending time looking around me and just enjoying what was happening. If you haven't been super involved in the student life at your school before now, get to it. Go to football games, go to homecoming and prom, go to movie nights or other school events. Those things are put on for you. Take advantage of them. Don't stress out too much over the little things. Grades are not the only important thing in your life and they don't define who you are.
College application season can be brutal. Be sure to take care of yourself in that time. I put all of my energy and focus into my applications during that time. I pulled countless all-nighters and ran myself dry. It can be really easy to push yourself to your breaking point during that time. When the decision season rolls around, it can get even worse, so remember this one thing: your college does not define your worth. A rejection letter is not the end of the world. You will end up in the exact school you need to be in. Just because you didn't get accepted for your first time, doesn't mean that you will never go there. You can go to another school, and then try and transfer.
Spend time getting closer to people toward the beginning of the year. Don't be afraid of opening up to new people and getting to know someone. Getting stronger relationships with friends earlier in the year can be so important in the long run. If you wait too long, it can be a lot harder to keep the friendships going.
Go on your senior trips if you get the opportunity. One of the best things that my school did was give us senior field trips. They were a great way for us to bond with your friends or even get closer to people you didn't know that well. They were a day that pulled you out of the classroom environment and brought you somewhere where you could just have fun and relax with your classmates. Some of my closest friendships started on a senior field trip.
Just because you are a senior, doesn't give you permission to be a jerk to the underclassmen. Be kind to them and don't forget that at one point, you were them. Use your "maturity" and seniority for good. Help underclassmen find their place. Help them be more comfortable in school. You are the leaders, so set an example for the classes to follow you.
If you're one of those people who can feel time going by too fast, just embrace it. Don't be too worried about the future. Be present and enjoy what is happening in the now. If you take too much time stressing about what is to come, you will miss out on what is happening right before you. Appreciate the time that you have, while it's still there.
Senior year is what you make of it.