I would like to preface this post with the fact that Pico's Unloaded doesn't fit in the timeline.
Firstly, the story starts with Pico's School 1.
Summary Of Pico 1
Pico is in class when his goth classmate Cassandra gets up and starts the rebellion. Pico exits and goes to the janitor's closet to get the gun. Pico goes to the neighbouring class and goes against Alucard. After beating Alucard, Pico gets the key to go to the other side of the hall. Cyclops is seen in the way and is quickly taken care of. Next, Pico goes to the girls' bathroom to find Hanzou, the ninja. After shooting with fate, Pico obtains the night vision goggles and with the goggles and gets the key from his teacher to go to the boiler room thing. After killing Cassandra's full form, Pico gets a few days off ( as known in pico 2 beginning dialogue ) and when he gets back to school, the "people from the ghetto" go to the school.
Pico 2
Pico is in class (after the few days of break) and the Ghetto Students appear (AKA ghettobots).Darnell explains to Pico where these guys came from, and they are clearly a threat. During what seems to be recess, Pico does some target practice with Darnell and the other kid when the Ghettobots appear and once again, Roc beats the living shit out of Darnell in one punch. The ghettobots warn pico that if he wants Darnell to live, they will have to visit them in West Philly. After school ends, Pico decides to hit the gym and go take the bus to the west side of Philly. He is quickly stopped due to his ethnicity, and negotiates until they agree on going to CC to see if pico has 'the soul'. Pico goes to Darnell's house for some practice, and afterwards he goes to CC.
The Pico 2 DJ competition game starts.
After Pico wins, he is let to take the west bus into West Philly. He is quickly stopped by Roc, who powerfully halts the bus. Roc rips the roof of the train open and gets down on the ground.
T-Phantom-T
Actually, Samurai Asshole is canon to Pico School. Tom Fulp has confirmed it when someone asked him if the aliens are connected to Cassandra and Tom Fulp said yes. Also, Samurai Asshole and the Pico School universe being connected goes back to Newgrounds Rumble because Samurai Asshole would team up with Pico and Nene to defeat Convict twice and isn't aggressive to Pico than the other character where he would just murder them.
So if you're looking for the real timeline of the Pico School series it would be Samurai Asshole, Pico School, Pico School 2, and the Pico 2 DJ competition game starts.