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Some seem capable of laser like focus on any job until it is completed. Others are easily distracted, jumping up from homework to do a hundred small, but suddenly urgent, tasks as the homework gets pushed further into the background. 
Like any other skill, the ability to focus can be learned and reinforced through practice. To improve your ability to concentrate, start by establishing a set time and place to study. If possible, study at the same time and in the same place every day. Establishing a routine gives study the importance it deserves and helps make studying a habit. Then, to keep yourself on task, set a small timer as you begin studying. Start by setting the timer to go off after 15 minutes. Until the timer goes off, give studying your full attention. If your mind wanders-and it will-pull it back to the task. Then reward yourself with something small: 5 minutes of solitaire on your computer or a trip to the refrigerator for a glass of iced tea. Time your reward, too-about 5 minutes should be sufficient. Then set the timer for another 15 minutes. 
As concentration becomes a habit, that habit will spill over into the classroom, too. You will be better able to focus on your instructor's words or on the test you are taking. If extraneous noises during a test still distract you, invest in a pair of earplugs to shut out noise as you take your test. 
The ability to concentrate is a necessary skill. Fortunately, it is a skill that can be improved with effort.