From your statement above, I see you are experiencing anxiety and the reason is peer pressure. If there is any other reason that you think is causing you anxiety, please do mention that also. However, feeling this way is completely normal. All of us at some points in our lives feel overwhelmed with too much that is going around us. If you feel like you're being a victim to peer pressure, then you must also find ways to not let it get to you.
Identifying the problem is the first step in dealing with it! Your studies are your priority. It is something that only you can work on and achieve results. Don't let the environment bother you in such a manner that it affects your sole purpose of going to school/college . Speak to your friends and deliver your point. You're an adult and they can't pressurize you into things. You have a choice of yours to make. A lot of time situations aren't in our control, but many times they are and I feel like for you it is in your control!
Do ponder up this and I'll be happy to hear from you again :)
One thing that helps me manage my studies in the face of anxiety is making a to-do list with a task broken down into multiple, smaller components. So, for example, if you have to finish studying one chapter for a test, which may seem like a lot, you can think of it as three smaller tasks: divide the chapter into three parts and at one time, focus on only completing one part. This tricks you into believing the task at hand is actually simpler than originally thought and you're more likely to at least start tackling it.
For peer pressure, I understand it can be difficult to say no to your friends. It can be even more difficult in a group setting. You can try talking to one friend about it, any one who you feel closest to or is the most understanding of the lot. It might be easier to put your point across in front of one person.
The problem lies on the fact that when I am studying one subject, I feel I am neglecting other subjects or fear that there is so much to be done in other subjects. So I could focus neither on the present subject nor other pending subjects. Is there any solution for the same?