With the Paris Museum Pass you gain free entry, without standing in line, and as many time you wish. Unlimited number for visit for 2, 4 or 6 days. It is cost 30 euro for 2-day pass, 45 euro for 4-day pass and 60 euro for 6-day pass. (Louvre- 8.50,Chateau de Versailles- 13.50, D`Orsay- 7.50).
Most museums and monuments are free for the under 18s, and grand discounts to 18 to 25 year olds.
I recommend activating your Museum Pass for Paris in the morning of first day in Paris to make the most of your day.