Hi K.!
I am a cosmetologist and have seen my share of headlice, believe me! If you like the school that your daughter is attending, it is not wise to pull her out over this, especially when it seems more logical that your older daughter probably had the problem first and passed it on to her younger sister! The fact that your older daughter had nits sounds more likely that she had lice first, since she already had the eggs laid on her scalp. Since you just found live lice in the younger one's hair without nits, she was probably just beginning to get them. It is a traumatizing situation for children and quite stressful for the parent who has to clean up the mess that lice leave behind, but it is extremely common with children and you can't protect them by pulling them out of their school.
You did well by using the Rid and should you find anymore, you can get a stronger solution via prescription through your pediatrician. Make sure that you contact the schools that both of your daughters attend and notify them of the problem! Chances are that they are unaware but will do something once informed. My oldest daughter's school has already had 2 outbreaks this year as well as my daughters' dancing school having 2 outbreaks since last July! Once the school was aware, they began sending each child to the nurse to be checked and the ones that had it were sent home until they were treated. They sent a letter home immediately alerting us of the situation and disinfected the school that afternoon. Luckily neither one of my daughters contracted lice-most likely because they never leave home with their hair down! It is wise to pull up their hair daily and if you use hairspray on them, it further deters lice from bothering to "force" their way in as opposed to a child who has their hair down, making them an easier target! It is a fallacy that lice are passed on from someone who is dirty, in fact, they only thrive in clean hair. They also can change in color depending on what color hair they happen to be residing in (black when in darker hair and white when in blond hair!)
Headlice can happen anywhere! Keep this in mind because you wouldn't want to cause more distress to your children by changing schools and having to start all over again, especially if they are getting a good education where they are! Good luck!