I guess the fair way to approach this would be to take the average YEARLY salary and cross it against the increase in hours worked.
Using your example:
Average salary = $51,000 yearly
Increase of 2% = $52,122 yearly an increase of $1,122.
Increase in hours worked proposed was 40 minutes per work day X 260 days of actual in school work (Roughly does vary across the nation) = 174 hours (Rounding up) extra hours worked.
$51,000 / 365 = $139.72 per each day of the year regardless of whether they're actually in school working.
$139.72 / 7 = $19.96 per hour.
$52,122 / 365 = $142.80 per day worked.
$142.80 / 7.7 = $18.54 per hour worked.
Conclusion effective pay rate decreases per hour worked. However, if the 2% increase is tied to the effective hourly rate of the teachers then the salary would be more than $51,122 as noted above. I did not read how the 2% increase was factored in. If factored into the hourly rate, then obviously the salary would increase and this whole conversation is moot.