You are interested in having a page in your School Loop site redirect to another site. This includes the homepage of your SL site.
Since SchoolLoop is not a domain registrar, SchoolLoop domains cannot be used to redirect to any external domain with a DNS redirect. Still, two options exist to obtain a similar result: a redirect banner, and JS redirects.
Using a redirect banner
The first option will be to create a banner explaining that the site has been moved, and with a link to redirect the end-user. This ensures the users know they are being redirected and why, and additional context may be provided on the page. This is not different from any other change that you may introduce to your School Loop site.
Note: If you are planning to discontinue using the SchoolLoop website, you can unpublish it. You can do this in the Site Center as described in Publishing School or District Site, by using the 'Unpublish Site' button.