If you can't select a school when generating a CTF or adding Education and Employment history, this means you have not yet added the institution to your site. Arbor won't create a list of schools - they come from what your school has added in.
You can see this in School > Linked Organisations > Educational Institutions. If the school of your choice is not on the list on this page, you will need to add this before sending the CTF by following this article.
When the school isn't listed here, you will need to add this as an Educational Institution.