To make sure all of your students are in a House, download a House list
STEP 1
Go to Setup

STEP 2
Choose Rosters

STEP 3
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.
STEP 4
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.