Such a dilemma, isnt it.
Chicken and egg situation where they wont let you hack out till you can canter, so there is a lot of pressure to start canter. But many older people like me learn to canter out hacking months before they do it in the school.
I probably had my first canter in a school about six months after starting to ride. I had ridden for two half hours each week. I had no proper instruction and was very put off.
So your first requirement is a suitable horse, a suitable place and a teacher who can teach.
Have you asked why the RI feels you are not ready? Whether the teacher feels you are ready to canter, or waits for you ask to canter, there are some basics to take into consideration.
I dont actually agree that it is easy to canter if one can ride trot with no stirrups. I always found riding with no stirrups very easy.
Before you start canter one needs to learn to do sitting trot with stirrups - which is (I was told) harder to do.
Most teachers ask one to sit a few steps of trot and then to ask for the transition to canter. So you need to be confident and stable in sitting trot, if that makes sense?
Then there is another factor, that of balance. One reason it is easy to learn canter out hacking is that one can choose a straight, preferably uphill stretch of track and there are no corners. In the school one has to canter round corners. And this requires the ability to stay straight on the horse as you go round a corner, with equal weight either side of the center, and without allowing one side of your body to slip forward.
Once your balance is fine and you can sit comfortably to trot and feel happy at the idea of cantering, then most of us would encourage you to try canter. But it is early days. 25 half hour lessons is very little saddle time. After a year's riding you are likely to feel very different. Many of us on this list did not canter seriously and enthusiasticly until we had been riding about two years. My personal experience is that if one canters early it can be a nightmare, but that if one canters late (however frustrating the wait) it comes very easy.