Whilst we will not make a specific recommendation, we can provide the following information to help you decide.
At the moment (June ’16) there are 8 models of ipad available, in 4 different colours. The school does not sell the 12.9” iPad pro, and discourages its use in the classroom. For school work, all models available in the school iPad shop have been tested and approved. Lots of students (and their families) also use the iPad for private activities such as watching movies, playing games or taking pictures. If you are planning for that to be the case, then we recommend a model with at least 64GB of memory.
The difference between the cheaper iPad Air 2 and the more expensive iPad 'pro' will not be noticed by many people. Although the ‘pro’ is equipped with more functionality and slightly better hardware, for school work the iPad air 2 provides more than enough performance. Most iPads will last for at least 5 years.
It is helpful to note that every iPad (or to be correct: every apple id) comes with a limited backup storage of 5 gigabytes. People who use their iPad for movies, music or pictures will quickly find this 5gb is not enough, and will find that the backup will stop working unless a monthly payment is made (starting at €3 pm). This payment can be avoided if the owner makes regular backup copies of the iPad onto their home pc or Mac.