The bus was travelling through Dahyan market in Saada province at the time.
The health ministry run by the rebel Houthi movement said at least 43 people were killed. The Red Cross said most of the victims were under 10 years old.
The coalition, which is backing Yemen's government in a war with the Houthis, said its actions were "legitimate".
"[The air strike] conformed to international and humanitarian laws," spokesman Col Turki al-Malki said in a statement.
He added that the coalition had responded to ballistic missile launches by the rebels, which he said had targeted civilians in the southern Saudi city of Jizan.
The coalition has insisted it never deliberately targets civilians but human rights groups have accused it of bombing markets, schools, hospitals and residential areas over the past three years.