Once you've enrolled in a college, you will be able to build up your Coach's Trust by performing well in games/practices. There are a total of 11 levels that you will need to build up in order to achieve the max level. Each level is earned by gaining the required points of the level. Points can be earned by completing practices and games. The lower levels will require 620 points, while the higher levels will require 1000. If you're playing on Freshman difficulty you will be able to achieve the max level before you complete your first season.
As long as you gain the required points when playing a season game, you will not have to play the practice session. You can check your Coach Trust in the Main Menu of RTG mode.