The Gate of Control, also known as the First Gate, is a gate that Iron Year Magisterium students must pass through in order to progress onto the Copper Year.
This gate can be found in the Hall of Graduates along with the other gates. It will only activate for a mage who has learned enough control over their magic to no longer be a danger to the world or himself/herself. When a mage is ready to pass through the gate, he/she can see a light illuminating the words Prima Materia on the archway. When a mage passes through the gate, they are formally proclaimed a full-fledged mage whose magic can no longer be bound by a Master.
Callum Hunt, Aaron Stewart, and Tamara Rajavi first stumbled upon the Hall of Graduates by mistake. Aaron passed through the First Gate which did not activate for him, meaning he hadn't been ready at that time. Some time after, their Master William Rufus decided that they were ready and had them pass the gate ahead of the other apprentices of their year.