Seda, They are both correct, but there is a difference in their meaning. It has been a long time since "we spoke" to our highschool friends. -In this sentence you are thinking of the "moment" a long time ago when you last spoke to them. It has been a long time since we "have spoken" to our highschool friends. -In this sentence you are thinking of the "long span" of time that has passed since you spoke to them.