1) most tablets do not come with a standard computer size keyboard. Most keyboards that work with tablets are in fact much smaller and do not lend themselves to proper typing for long term use. This is a problem even when you can find a keyboard that works mostly well because the bottom line is now you have 2 devices you have to put away in a cart, track, and not break. Two devices that will need to be replaced. Oy. As a result, students loose out on computer skills. In addition, note taking itself is a burdensome task on a tablet.
2) the touch screen is an amazing feature making it very kid friendly, but on the other hand, since most computers do not have this it impedes a child's ability to learn to use a mouse
3) Many tablet are unable to multitask which is something that is often needed when a child is working on a paper or doing an activity for school.
4) Tablets have very definitive life spans, often much shorter than a computer, such as a macbook.
5) The iPad does not have a standard operating system like Windows on it nor does it operate in any way similar to a desktop or laptop. Students will not become familiar with basic navigation of files that are essential.
It is for these reasons, that I veto purchasing tablets for schools en masse. It might be that a few could be useful for students to utilize to make videos in older grades or as devices to listen and watch flipped videos, but as a main computation device, they don't fit the bill. They are simply not laptop replacements.