I found out that this is the minimum score you need to beat the game. And yes, I do need a social life.
All main story missions.
No side missions (not even the Big Prank).
No collectables except for the first two radio transmitters which you can't avoid.
Played High Striker four times to beat Carnival Date.
The only clothing I bought was the Aquaberry Vest for The Eggs.
No bike races (except the two story races) and no go-kart races.
No errands (except for Petey's errand tutorial which doesn't add points to your score).
No classes (I failed Chemistry 1, the only required class).
Basically since I had finished the game numerous times with 100%, this challenge was the opposite.