Police putting witness in police car?

I'm 14 years old. I was on my way to school when i saw a fight between 2 13 year olds. This happened at the bus station. The police was contacted. Once the police arrived they put the 2 13 year olds in handcuffs in seprate police cars. The police then came to me and said they need to talk to me about what I saw. He took me to his police car opened the door and told me to sit in the backseat of his police car. It was a ford crown victoria. There was window bars, a cage between the front and backseat, and the backseat was vinyl. I was wearing a pair of those Adidas skinny soccer sweatpants, hoodie and Hightop shoes. Once in the car the officer got in the front. I asked why i had to sit in the back and he said its for privacy reasons. He questioned me for about 20 minutes then left me in the car for 20 minutes to write a statement. After he said he will drive me to school. After that he told me to get out of the car and he said he has to handcuff me since ill be riding in a police car. After that he place me in the car and he went to talk to another officer for 10 minutes then got in the car and drove me to school. The drive was 15 minutes to my school. My knees were against the cage the whole time and my wrists hurt after he put handcuff on me.

My questions is…

1. Why did i have to sit in the backseat of the police car? Why did he need to make me sit behind a cage. He could have used a non caged car.
2. Why did he handcuff me to drive me to school?
3. Why did he have limited legroom in the car?
4. Why did i have to sit in the back to for questioning? Is privacy true?
5. Why did he allow the handcuffs to hurt and my knees against the barrier? Is there any way to sit for more comfort?
6. Does my adidas sweatpants have germs from sitting in the car. My friend told me criminals do nasty things in the back of the car.

Added (1). This is a picture of the Adidas sweatpants.

It was my first time in the backseat of a police car. It was kinda scary expecially with the cage and the handcuffs but little bit fun.

I do not think he was right in making you wear handcuffs. That does not make sense to me. If that was the condition for driving you to school, I would refuse and walk instead. They can only handcuff you if you are under arrest, and you were not.