Some can read really well and some have a harder time learning to read. There are just lots of ways that kids differ. Kids can also differ in how energetic they are and in how their mind works. Some kids don't seem to have very much energy - they just like to sit around. Other kids have so much energy, though, that it is very hard for them to sit still.
Some kids are also able to really concentrate and think about one thing for a long time.
For other kids, though, their mind sort-of jumps from one idea to the next. Having all these different ideas can be great, but when you have to focus on just one thing at a time, it can make things hard. Sometimes kids with so much energy and so many different ideas need some help being able to sit still and focus on one thing at a time. One of the things that can help a lot with this is a kind of medicine. What the medicine can do is make it easier for you to stay in your seat and pay attention when you need to at school.