Bismillaah-ir-Rahmaan-ir-Raheem

Wa 'Alaykum-us-Salaam wa Rahmatullaahi wa Barakaatuh.

Attending a gathering or taking part in an activity that invloves listening to music is blameworthy and condemned in Islam.

A verse of the Glorious Qur'aan states:
(Satan said to Allaah when Allaah told him he will be accursed till the end of time because he refused to prostate to Adam).....He (Satan) said...." If you grant me respite (time to live) until the Day of Qiyaamah, I will surely gain control of his(Adam's) progeny (and lead them astray) except for a few individuals (whom You protect from me)." (Accepting his request) Allaah said, "Go! Whoever of them will follow you, then Jahannam will definitely be ample punishment for them. And fool (destroy) as many of them as you can WITH YOUR VOICE....." (Bani Israa'eel, 62-64)

Mufassireen write as an explanation to the phrase "WITH YOUR VOICE": It includes MUSIC, and all else that calls to sin.

Even though, you do not attend these classes for the purpose of listening to music, you will not go unaffected. The harms that music cause to the spirituality of a person are overwhelming.

The Messenger (sallallaahu 'alayhi wa sallam) said:
Certainly music creates hypocrasy in the heart just as (effectively as) water produces plants from the earth." (Abu Dawood)

It would be best for you to find an alternative way to keep fit and healthy as it is also Islamically advisable to do so, on condition that the bounds of Shari'ah are not transgressed in the process.

And Allaah knows best.

Wa Billaah-it-tawfiq.

Wassalaam