Sister attending a school dance.only have foam earplugs that block 33dc. are those okay to use or should i buy other ones?how loud are schools dances?

Sounds fine: Common foam ear plugs are fine for a school dance. The loudness of a school dance depends on the sound equipment and on how close to the speakers a person is dancing. One problem with ear plugs is that she cannot hear her friends talking with her... so she may be taking plugs out and putting them in, off and on during the dance. Most kids don't wear earplugs to school dances.