The Gregorian Chant Network exists in part to organise Chant training. Our approach is to organise events open to all, which are large enough to allow different groups for 'beginners' and 'more advanced/experienced' singers.
Chant training is available from a number of groups and individuals, however, who would be able to give a workshop to an individual schola. Here are some we know about:
The Schola Gregoriana of Cambridge
The Canterbury Gregorian Music Society