I do think Mom will get angry. Concerned, yes. Worried, yes. Anxious, yes. She'll say eleven is too young to date or have a boyfriend. She'll say school and learning is more important right now. She'll say wait a few years and let her daughter grow up a bit.
She'll ask serious, embarrassing questions like, are you planning to have sex, are you using contraception (condoms, spermicide) and do you understand the age of consent is 16 in most states. She'll ask you to stop seeing her daughter, with the penalty of being arrested for a host of offences such as Contributing to the Delinquency of a Minor, Harassment or even Statutory Rape.
At 15, if your parents allow you to date, that's one thing. Going behind a parent's back and dating an elementary school student is not in your own best interests. You are much too young yourself to deal with the serious consequences of dating someone four years younger than you.
How to proceed from here? DON'T.