- Find the Oldies: Visit Google Contacts and search for outdated names - check under "contacts" and "other contacts"
- Purge Away: Select and delete any old contacts you no longer need.
- Turn Off the Setting: Go to Gmail > Settings (gear icon) > "See all settings" > "General" tab > Create contacts for auto-complete and set it to I'll add contacts myself. This stops Gmail from auto-saving addresses of everyone you've ever emailed.
If you want to bulk delete contacts you will still need to select them individually (do 10-15 at a time) and then click the three verticle dots and select "delete". It's possible to sort/find old students by typing in the last part of the email indicating class year (ie. .23). See screenshot for example of a sort-->select-->delete.
Note: Because Google sometimes thinks it is smarter than you, removing contacts from "Other Contacts" might not completely prevent them from appearing in suggestions, but it significantly reduces the likelihood!