He's growing up so fast!

Prince George was escorted by dad Prince William to his first day of school at Thomas's Battersea in London today, and the little royal looked ready to learn in his adorable uniform. The 4-year-old greeted his principal and was every inch the gentleman by giving her a handshake, and he didn't seem that nervous about the big milestone!

George's mom Kate Middleton was unable to come on the outing, as she's struggling with severe morning sickness after announcing she's expecting her third child.

"Unfortunately The Duchess of Cambridge remains unwell, and will not be able to accompany Prince George on his first day of school. The Duke of Cambridge will drop off Prince George this morning as planned," a palace source said in a statement.

We're dying to know if Prince George and Princess Charlotte are getting a brother or a sister!