Possible Clues:

• How one might work out result of an extended parliamentary vote
• Arithmetical process
• Somewhat hard math subject (2 words)
• Arithmetic exercise ... or what literally occurs in 20-, 33- and 40-Across?
• Protracted disunity that was once more common in primary schools
• Faculty backs learner doing badly in mathematical process
• Arithmetic exercise
• 25 into 435 = 17.4 yearn for what is in the army
• Lengthy split in schoolwork?
• Extensive calculating needed to create pine border