On the Site Admin > Plugins > Authentication > Manage Authentication page it allows you to specify an alternate login page (which could be your school website) and gives advice on ensuring the correct fields are sent from the login form you create on that page. Its basically just setting up an html form on your page and ensuring that goes to the right place with the right info - and doing it securely so username/passwords are not sent in the open.
In fact, for your isntance, I'm not even sure you need to specify that alternate login, provided your form calls the moodle login.php and sends the username and password, you should be fine and would keep the standard login page as well as being able to login from the school website (note: haven't tested that, but I know the site I have at the moment has an external login page, as well as a SAML Single Sign On link and the standard login page also still works - when the saml=off flag is added)
R