Google Docs (Spreadsheet) tells me that this (custom) formula is wrong.. in Excel instead works:
=AND(MATCH(CONCATENATE($A2;\" \";$B2);Allievi!$G$1:$G$1204;0); $
You can reference the sheet and cell indirectly.
=INDIRECT("SHEETNAME!"&"CELLRANGE")
INCLUDE THE QUOTATION MARKS
For your formula it should be something like
=AND(MATCH(CONCATENATE($A2;" ";$B2);INDIRECT("Allievi!"&"$G$1:$G$1204";0); $L2="Facilitatore di classe")