Your mom is right! You are already building credit by having student loans and paying them off. In fact, your credit history started the day you applied for your student loans! Paying them off on time is making sure you add positive points to your credit report, which will in turn result in a good credit score.
You don’t HAVE to have a credit card to build credit. It certainly helps, but it’s only one of many ways to build good credit. Follow me, my children, through the following links to learn aaaaalllllll about it: