The only rationale I can come up with is your mother is afraid you'll get caught up in making friends (some which will be "bad") and jepordize your academic success. That said, it's pretty bizarre because you cant know right off the bat who would be a good friend for you without getting your hands dirty and knowing people. Additionally, if you have only 1 friend and refuse to make others by trying to obey your mother, you'll be viewed as a loner and people probably will come to ignore you. In other words, the opposite of what you want.

If I were you, I'd meet different people at school, don't call them friends since you don't yet know them. Once you have more experience with them, figure out which is your best friend and label that one your, "friend". The others are still your friends but when you're talking to your mother about them, refer to them as acquaintances. Alternatively, present all of them to your mother as friends and let your mother realize you are capable of having multiple, "good" friends and still succeed academically. If she doesn't like that... well, if I were you, I'd ask what her rationale is and if it isn't bizarre, I'd keep the friends.