how to convince your mom....?

i just started jazz classes last year, once a week, and im already really good. but my teacher knows i can get even better with ballet classes too. shes really pushing me to do ballet and i really want to do it too. but the ballet class recommended for my level is twice a week and my mom doesnt like that. she thinks 3 dance classes a week is too much since i already have mathnasium (tutoring) twice a week and piano lessons once a week. and i have schoolwork. she thinks i cant handle it but i know i can!! im doing dance recreationally but i want to be good. my mom doesnt want me focusing all on dance. but im in love with it<3 how can i convince her to sign me up for ballet also?