While you are a student, you should be focusing on the assignments you are given (whether you like them or not). A part of improving is developing the skill to be able to focus and solve problems you may NOT like. Your primary goal should be to complete your studies as soon as possible and building skill on the way. However, if you have spare time, you canuse the following sites to practice your skills.
overthewire.org
Games that help you improve your BASH skills.
CodeChef.com
It is freely accessible to anyone. There are a lot problems to solve, categorized by their level of difficulty. The problems there will keep you busy for a lot of time.
coderbyte.com
It is freely accessible to anyone. There are a lot problems to solve, categorized by their level of difficulty.
It even includes a GUI to write the code.tausiq blog
This is a wordpress blog also freely accessible to you. It has a list of many easy problems to solve (accessible here).
codility.com
This is a very well known and respected website among the professionals, it is even used by companies to check their employee skills. You can find a lot of problems to solve there.
You can find some solutions at https://codesays.com and https://www.martinkysel.com
If you want more difficult problems, you can read this post.
Great site to teach you how to build a lot of things