Personally, Gokudera thought the boxing club would be good for Tsuna... if he actually learned to hit back. Gokudera didn't want to see Tsuna just get beat up more under the guise of learning to defend himself.
"Bye, Kyoko," Gokudera at least managed to say to the girl before she ran off to class. He was already going to be late to his own class... so he just decided to skip the rest of it and headed up the the roof to hide think.