5 year old daughter misbehaving in kindergarten

I have a 5 year old little girl. She is great at home but requires a lot of attention i work 5 nights a week and my husband works in the afternoon... even after that we manage to dedicate time to her... when we are at work my mom takes care of her.. she is really good at home.. she listens, plays, and does what is told... at school is the problem she hits kids she wants to play all the time she doesn't listens to the teacher... the teacher says that she is really smart in her class the is 10 kinder kids and 10 1st grader kids... she is up to the speed of the first graders but then she says that is too easy.... i noticed at home that she forgets a lot of letters or numbers easily.... she is easily distracted... other than that she is great... they have a chart of behavior in class from green (in the middle) to blue (good) to red down (bad) she often goes to brown then red.... i do the time out thing... and i do the time out of her favorite toys because that seems to work on things at home but i don't know how to help her in school??? when she does good in school and she goes back to green or yellow towards blue i reward her..... she is an only child.... so it hurts me to see when she doesn't do well in  school.... Please help me give me some advise what I'm i doing wrong... or what should i do to help her.....