My school issued us Apple MacBook Airs this year and I was curious if I used my own personal Apple ID to FaceTime my friends would my school be able to track what I have said?
According to Apple's iOS security whitepaper (issue March 2017):
The audio/video contents of FaceTime calls are protected by end-to-end encryption, so no one but the sender and receiver can access them.
Your school would have to actively engage in a network attack, which seems highly unlikely to me (and probably violates federal laws, depending on where you're from).
That being said, since the devices are school issued, they might've been able to record your calls in other ways:
By hijacking your microphone / webcam;
By replacing FaceTime with a malicious imposter application;
By intercepting traffic before it leaves the device.