[ghostdog74]

you can use awk

awk -v file="Prac1" ' BEGIN {  # get all sids of Prac1 into array
		while( (getline line < file )>0 ) { 
			if (line ~ /^#/ ) continue #skip comments 
			mprac[line]
		}
      }
      /^#/ {next} #skip comments line in Students file
	{ 		
		if (!( $1 in mprac )) {
			print "Sid: " $1 " not in " FILENAME
		}
	
	}
	
' "Students"