Sorry, no. That is not a reason to skip school, in my opinion. Maybe if one of your immediate family members was being celebrated at the event... *maybe* then. But just as a general, "oh, my team won... time to party"... No.
I think it sets a poor view of the importance of school and how to set priorities. Particularly for a student who has mediocre grades.
I have a 16 year old 11th grader myself... and he would not be allowed to miss school for an event like that. Skipping school is generally frowned upon in our home. The only "sanctioned" skipping here is the last day of the semester when the kids have already finished their final exams (they move classes around on the last 2 days each semester, on the block system, so that they have 2 finals on the next to last day, and 2 on the last day of the semester). And only then b/c the teachers really don't want the students there. Once they have finished their tests, there is nothing for them to do except be babysat, really. They can't do anything for a grade and the teachers want the kids gone so they can focus on getting tests scored and grades sent in.
Otherwise, no skipping.