if i loss a challenge will my xp decrease?

Answer 1

if you challenged low-courses xp player and lose
you'll lose 32xp - xp for correct answer
e.g.: your correct answer is 3, your xp will decreased by 32-3=29xp

if you challenged high-courses xp player and lose
you'll lose 1xp - xp for correct answer
e.g.: your correct answer is 3, your xp will increased by 1-3=2xp

Answer 2

Sometimes it will. It really depends on the opponent.