Example of a training lance:
Instructor - Veteran MW in Medium or light Heavy 'Mech
MW Student - Green MW in light 'Mech
Vehicle Crew (various, from Ultra-Green to Veteran, average Regular) - in Heavy tank
Recruit platoon - various Green and Ultra-Green recruits with some Small Arms skill, can become something else once they have enough points.
The average weight is low enough to call it a Light lance (particularly if you're using vehicles at half-weight), and the instructor and heavy vehicle are usually enough to pull it off with minimal help from the MW trainee, while the infantry hide. That's in the event that it does get sent into battle; usually they'll remain in reserve and just run simulations.
As long as there's one Veteran officer in the group, and you dedicate the lance to Training, you can train up Green or lower troops to Regulars at +1XP per week. The XP cost to boost a Green up to Regular depends on your AtB settings, but if the cost is 10XP for gunnery and 10XP for piloting, that's 20 weeks of training in the field at normal deployment costs to go from Green to Regular. In this example, I've got 1 MW, 1-2 vehicle crew members, and up to 28 various "others" in training each week, with enough total punch to actually survive a mission. In about 10-20 weeks, I can rotate a few of the infantry, because they'll have enough points to begin some other role and accumulate points from that instead. The MW and vehicle crew trainees will take the full 20 weeks before rotating.
As pointed out by Schugger, if they're forced to fight, you can either refuse the mission for a -2 hit to your contract score, deploy and then surrender for a -1 loss, or else attempt the mission and then bug out for the loss if/when things start to go sour, but at least you tried.