Whether you are in university or school, studying multiple subjects becomes difficult. But, it has been forever like this. Your seniors have done it. Some even would have scored straight As in every subject.
Some of the A students may be exceptional. But, most are average. Hard work and consistency coupled with effective study techniques earn them top grades. Here are some proven study tips for retaining concepts in your memory for long term, and acing your exams. 

Organise 

If you notice, the index page on your textbook makes it quite easy to land on a specific chapter. The information is organised, and that makes it easy to go to the chapter rather than flipping the pages.
Keeping track of assignment due dates, missed classes and exam dates and noting questions to ask in the class will help you get a better sense of how many things you have to do. This will help prioritise tasks and manage time. 

Keeping devices, stationery, and water nearby saves you the effort to find them during the session. It may also break your flow.   

Plan 

If you have an exam next week, preparing at least a few days before rather than just a day before will be more rewarding. Planning enables you to organise resources and allot time effectively. 

Be regular in class 

Passing the exam should not be the only reason you attend classes. Discussions bring various perspectives. Participation in class engages with the subject. If you are skipping classes because you would like to do it on your own, you may miss a lot of things in the class. 

In the class, your course material is covered steadily which gives you time to absorb and come up with questions. 

Take and make notes 

Noting down important and difficult topics keeps them handy for reference. Taking notes also keeps you engaged in the class.
You can refine your notes after class. Highlight, append visuals and additional information, and simplify by using diagrams and flow charts.
If you self-study, get into the habit of making notes. Once you have taken down notes, you will save time by referring to your notes. And condense them later to keep only the relevant things.
 

Ask help 

Your teachers and resources. If you get stuck, or find something difficult to understand, approach your teachers. 

Your teachers have been instructing for years, they know where students are likely to stumble. Your teachers will direct you towards the right approach. You can also get instant feedback for your solutions. 

Spaced out learning 

If you spread your learning session within several hours rather than a long session, you will process more information, hence retaining more information.
Spaced-out learning allows regular breaks that interrupt the monotony and keeps your rhythm going. 

Test your learning 

The best to evaluate what you have learned is to test it. Solve review questions and problems at the end of the chapter to assess your learning. When you solve these questions, you get a clear idea of the effectiveness of your efforts. 

Try quizzes on the internet to brush up on your knowledge.