
We don't really have an explicit chapters when He Tian leaves, but I think 445 kinda gives an idea that he already left. He didn't answer to Mo message's so Mo thinks he forgot to set an alarm, after that the rest of the chapter are only with the adult version of them. Or maybe in the future Old Xian will give us a proper chapter with what happened.

The school chapters are in the past of the boys. They are supposed to be in the last year of middle school, but I think is a plot hole and Old Xian later changed to High school so they're not that young. All the alternative chapters ( Christmas or Chinese New Year) are with them as adults. Form 452 to now we see the lives they have right now, Mo at the car shop, Zhan after college trying to get a job, He Tian came back and Jian was supposed to repeat the High school or is in college to, but much later than Zhan.
Sorry I yapp to much, I'm really invested (⊙…⊙ )
The photos are covered because, probably, Mo or hated him for leaving and didn't want to see his face anymore or missed him so much that looking at his face was painful. Mo is cold to He Tian now, but we know from the Christmas chapter that he cried all night when he realized that He Tian is back.