As a teacher, there are many options available for you.
There is the evergreen Ms. Frizzle from magic school bus, if you don't want to scare your students.
Or try doning the costume of any mad scientist characters like Doc Brown from Back to the Future.
But you are at the liberty of choosing your own costume, the there is costume idea generator.
Where to buy Costume
Then there are various blogger that make DIY Costume Guides for Halloween, which can help you in saving money and doing your bit for saving the environment.