In the days of the Old Jedi Order, the practice of multiple Padawans learning under a single Jedi Master was discouraged, due to their apparent prediliction to defect to the dark ways of the Sith. However, this discouragement was banished in the early days of Luke Skywalker's New Jedi Order. In fact, several early Jedi who eventually formed this New Jedi order never had the "rank" of Padawan, since there were no Order or Masters to train them. Plus, the term Apprentice was used more frequently until recent times, where now the terms Padawan and Apprentice are almost evenly used in the Order.
The problems of this new system, where one Master trains multuple students, are not ignored. However, until such time where the Order is strong enough to allow Knights to train Padawans, it will simply need to continue.
In the Old Jedi Order, a Padawan would follow and help his/her Master everywhere, unless seen unfit by the Master, from simple tasks to dangerous missions, such that he learned from experience and personal guidance. To balance this method in current times, Padawans are encouraged to spend most of their time sharing what they learn with each other, and helping in each other's training as much as learning directly from their Jedi Master.