It all depends on the school you attend, how well-respected your school is, and how focused you are with your gadget.
You should not have the gadget if you are in elementary or middle school. It is unlikely that you have the ability to keep the gadget in your bag. Instead, focus on the lessons and your teacher.
It really depends on which high school you attend. I work in a school that allows you to bring your own devices, and I wish they would ban them. Our school has a population that is very wealthy or very poor. This means that not everyone in the school has the gadgets and the ones who do not have the devices will not follow the instructions of the teachers about when they should be taken out and when they should be put away. My school does not require that disruptive students put away their gadgets at the appointed times. These schools are not equipped with the latest technology and have a younger student population. Teachers would prefer that you leave your gadgets at home. They do not help your learning.
You can take any device that you think will be helpful in your studies to university. My phone, e-reader and laptop were always with me to class. The hard sciences and mathematics were my majors. We do a lot programming, so the laptop was a great tool. It was also possible to coordinate with your friends and work together on the same google document. Because I could get most of my text books in.epub and.pdf format for free, I bought an ereader. I also kept my books on the reader. My phone was used to coordinate with my study group.