Bullying on the school bus?

Today was my daughters first day on the bus. She sat next to a girl who was being spoken to rudely by some older boys and my daughter told the older boys to 'cut it out' or leave her alone or something to this effect. The older boys then told my daughter to shut up and continued teasing the first girl. I am conflicted as to what advice to give my daughter as i was proud she stood up to the older boys but at the same time my motherly nature wants her to basically ignore the situation so she can be safe as i know children are more dangerous in these times. any advice from parents who have been there?