Some tips to help:
Buy some nozzle cleaners and clean nozzle which should be about size 3.
Make sure gasses are correct pressure and balanced. If you are continualy having problems setting gas pressures, then ask college technician to check or change the regulator diaphragms.
When setting up the flame, allow about 50mm acetylene flame and introduce oxygen slowly until flame is neutral and bright (slightly rounded on inner blue tip).
Make sure work is clean and flat
Introduce flame and hold torch close to surfact of lead (if the torch is too far way, you pick up oxygen in atmosphere and produce an oxydising flame) - too close and you get moulten lead in your face, but it must be close.
Make sure angle of torch is 30-45 degrees which is at same angle as filler rod.
I accept that lead work is now irrelevant to most occupational areas of plumbing, but the training will give you good experience in using flamable gasses and developing the skills that are probably the most transferable in plumbing - if you can do it in a workshop you can most likely do it at work in the case of lead installation and fabrication - which cannot be said about systems and other types of plumbing work.