Q. Is it permitted to attend or to teach dancing classes during Sefira? (For women only in an only girls schoo)

A. Mogen Avrohom (493: 1) rules that non-mitzva dancing is prohibited during Sefira days.
However, Igros Moshe (O.H. 3: 87) permits teaching instrumental music when it is done as a professional duty and not for enjoyment. See Naharos Eisan (p. 252) in regards to permitting singing and playing music for children in school. See question 1305 and 1302 in regards to classical music and other solemn and sedate music. However, some Poskim point out that dancing is by its nature more joyful than singing.
Horav Shlomo Miller’s Shlit’a opinion is that one should avoid if possible attending or teaching dancing classes during Sefira,

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a