Could this be bipolar disorder?

My sister has strange mood changes in school, where at one point should will get extremely depressed and cry because of it. She would feel horrible and feel like theres no way out. She is sent to the nurse everyday because it comes on randomly usually without a reason. I know people with bipolar disorder experience highs and lows, but shes sad randomly without reason get really severly depressed and helpess. Could this be Bipolar disorer?