ADD: I don't know how I missed the part about "her dad is mean to her", but you REALLY need to know EXACTLY how he is "mean". Withdrawal like this around a parent (or anyone the child used to be comfortable around) is a RED FLAG.
Not to be alarmist, but this could be anywhere from mean like yelling, etc., or mean like inappropriate. Somehow you need to get her to tell you what exactly is the "mean" behavior. Reassure her that you will NOT get mad at her and that you need to know so you can help things get better and stop the mean stuff - but that you need to know what the mean stuff is.
Original: Talk to her teacher. Talk to your pediatrician. This doesn't sound normal.
Please don't say she's lazy. If she's truely depressed, then it's not lazy, it's lethargic.
Good luck!