“many times i walk in the street and people oppsite gender say hi/ goodmorning et cetra should i respond?”
I would say that it would be a good idea to respond, if only not to appear rude.
“how friendly do you have to be to classmates of the opposite gender?”
Now this is much more of a question. One should not have opposite-gender friends, but on the other hand we can’t make a chillul Hashem. Therefore, I would say to be polite (say hello, hold the door open, and in general show you acknowledge he/she exists), but not friendly (have a real conversation with him/her, ask them how their vacation was, etc).