It is an easy thing to do, to fall in love with mice. They are cute animals. However, before you go out and buy your first mouse, you should always remeber that in spite of being easy to care for, owning a mouse is a huge responsibility and will take up your time. So, you need to consider the following points:
This is certainly something you need to find out beforehand. You don't want to end up in a situation where you are forced to find a new home for your mice for example because your parents will not tolerate your new hobby. Best situation for you and especially your (future) mice is that the whole of your family is excited and happy about your new pets. (I have been in this situation, not very pretty.)
This means: can you be certain that your mice will get good care if you are ill or away from home for a few days? If you are allowed to get mice on the condition that no other family member than you need to touch them or even think about them (it is you who will take care of them, by yourself), you will have to think beforehand. If none of your family is interested in the welfare of your mice, who will take care of them when you can't do it? You may think now that you are going to stay home enough to take good care of your pets, but can you be sure? You can get sick and be unable to even stand up, not to mention feed you mice, for many days. Your class might go out on a trip for a few days... Your friend may agree to take care of your mice when you can't, but you should be able to trust your family to be there for your pets.
It does pay to discuss early in advance on the question about who will take care of your mice when you are not able to do it yourself.
A mouse will not thrive in just any small box. Mouse is small, but it does need space. You will have to be prepared to get your mouse a home of adequate size. A home which is not only roomy, but safe for the mouse and easy to clean. Furthermore, you need equipment. Food bowl and water bottle are essential, but not the only things your pets need. Mice need nesting box (or other type of nest), a large and safe wheel to run in and other toys to play with.
Buying your mouse's home with all equipment can be fairly expensive, although you may find cheaper used stuff as well.
You will have to look after you mice every single day. Basic care consist of looking after that the mouse has food and fresh water. Food bowls should be washed daily with hot water (mice tend to pee in their food bowls) and water should be changed daily - rinsing the bottle carefully as well. In addition to this basic care you need to interact with your mice - they are pets, after all. By interacting with your mouse, you will get a perfectly tame and nice pet. Furthermore, due to the small size of a mouse you need to check its well being every day. Ailments can develop very fast and you have to know when your pet is not feeling well in order to get proper treatment (medical, taking the mouse to vet) as fast as possible.
Oh well, this may sound like a strange question on a mouse site - you being here shows you have some interest in mice!
Nevertheless, it is extremely important that you get all the information you need and to get the information before getting a mouse. Not knowing better is no excuse for bad treatment. Information is there, it can be found - it is up to you to be active and find it. No not rely on information given out by a pet shop, especially of the shop keeps its animals in crowled, dirty environment. Also, no not rely on only one source for information - this includes reading just this site.