To make sure all of your students are in a House, download a House list
Go to Setup
Click the "Add or Remove Students" button for the House you'd like to edit
If switching up the teams for a new competition, consider creating a new roster instead of editing the House roster. This way you can view data for the previous and the new House as separate groups.
Click "Add Students" and Choose the Grade the Student Belongs to
STEPS 5 & 6
Select the Student(s) and Click "Add Students"
House totals are updated automatically when you update the House roster. In other words, adding a student to a House will add all of the points the student has earned (since the House Points Start Date) to the House total, and removing a student from a House will take away that student's points from the House total.