Is child abuse legal?

I swear, I have never had school problems with my youngest before, but shes jumping head first into them. Shes failing 3rd grade. Shes not doing her work, not turning stuff in, not bringing stuff back.
What do you do with an 8 year old?
Beat her?
Seriously, I am grounding her, & going to sit on top of her at night, making sure she gets work done & am assigning her extra work as well. Something has to happen, or she will fail the third grade. Failure is not an option, not from laziness.
They are doing a vision/ hearing screening on her, trying to see if there's a physical reason for her having issues.
Teacher says shes lethargic in class, puts her head on her desk a lot, I guess I'll be adjusting bedtime as well. Come to think of it, shes been having trouble getting to sleep. No tv now. Her routine is going to be school, homework, short free time, dinner , bath & bed.
Any ideas? I welcome them.

On to the older one. Shes known about her science project since October & has yet to get it done. Its due the 22nd......
Sheer laziness. Time to shut the computer off too, make her use it only in the living room for class. I've had it with this crap. There's no excuse.