I'm confused. This is your first day in general or your first day returning after the two weeks?

They thought you were lying -- you can't build a school in two weeks!

You give two weeks notice when you quit and do not tell anyone ahead of time - even if you consider them a friend and the conversation as private. Never more, never less notice. It’s not that uncommon for the worst companies to fire you if you give longer notice. It’s a stupid over reaction to fire an employee who is leaving, but some bosses take it personally. Most larger companies have a policy of giving you two weeks notice or at least two weeks salary if they fire you. So you are not out anything if they fire you when you give two weeks notice. They know that, so they will seldom fire you for giving two weeks notice.

pretty sure that's illegal to fire you.

When you start a job, and that company is going to invest time and money training you, they don’t want you leaving for two weeks early on. Your commitment to your job is more important, and unless you have experience in construction, you, like the vast majority of these idiots who fundraise to go on a trip like this, are pretty much useless and the money you are going to blow on airfare would be much more useful to those actually building the school.

You are not very smart. You don’t start a job and expect a 2week vacation after a few months. You deserve it