A few reasons why forcing may not be such a good idea. The more you hold the One Power the more you want to channell. The more you channel before you have learned self control the more likely you are to either burn yourself our or worse go off like a bomb and hurt people near you.

Given that a lot of girls who come to the Tower are young, at least under 17 when they start they need to develope emotional maturity to match their channelling abilities. Do you want a teenager to develope her full strenght before she has learned to control her temper, get along with others. A normal teenager might just throw a tantrum. A novice/Accepted might lash out with the One Power without ever meaning to.

Secondly a lot of the training an novice does is not connected to the One Power. Its more like a high school education in history, politics, law, georgraphy, poeple and customs. Then they graduate to Accepted and study at their own pace, a bit like university. If they get too strong too fact in the One Power they might be tempted to ignore their other lessons which can be just as important in later life.

Think of say a football star at school who does not need to study too hard as he/she will get a pass because of their abilities on the sports field. That's great while at school or colleague. What happens when they go out into the real world and are not sports stars.

Unless the Aes Sedai are going to become one power bullies they need to know about law and customs when they are out in the world and have to deal with people.

Lastly being an Aes Sedia is a way of thinking. I would imagine that the leaders of the tower want to make sure that recruits are on side so to speak. Some might call it brainwashing but some professional groups also give you a sense of the history of the profession ,what it is suppost to stand for etc.