The answer is Sir. The young trainee was an apprentice squire. It was the responsibility that each lord train their own knights who in time of need were to support the king. There were occasionally titles such as master of arms, armorer, master of horse, and champion. All of the above were responsible and took part in the young lads training but the knight who sponsored the youth was titled Sir. During times of extreme threats the trainees would all be knighted and sent into the field to repel the danger. Most of those were ill trained and not yet mature so they became the majority of the battlefield dead.
In the Lord's or King's Castle there is a 'Regiment' and they would train the young boys to be knights. This would be sort of like boot camp now in the military. And just like boot camp now, there are different people who teach different skills. Horsemanship would be one class with one set of teachers, archery would be another and swordsmanship another ans so on.