What does he mean by "get off your ath"?

Native speaker’s answer
Rebecca
So, here, to make it more "kid" appropriate, he censored the word "ass" to "ath". The word "ath" also looks like "math" without the "m". He's telling them to get off their seats and do math. Ex: Get off your behind and do some work! Ex: You can't just sit around all day!