Trade-in
You can trade-in as many qualifying devices as you wish - either in-store at a physical Apple Store, or using the online trade-in process:
Apple Trade In - Apple
Each device is assessed its own right. If using the online trade-in, you will be sent a shipping kit for each device. Devices are returned, assessed and a credit value assigned. The trade-in value is returned to you by way of an Apple gift card for each device - and these can be used to purchase whatever you wish with Apple.
If a trade-in is initiated during an online purchase, you will usually pay full price for the item - but will receive a refund against your credit card upon subsequent return (and acceptance) of the item that was returned for trade-in.
You should note that when combining an online trade-in with purchase of a new device, you can only trade-in a single device. As such, for multiple devices, you must either complete a stand-alone trade-in for one or more of your devices prior to purchase - and redeem the Gift Card balance during a subsequent device purchase - or complete the trade-in of multiple devices in-store. Many choose to complete their trade-in in-store anyway - as doing so removes any uncertainty as to the trade-in value or acceptance.
Educational Discount
Student Discount programmes from Apple, where available, require that eligible students are enrolled in a recognised higher education programme within the country of purchase.
In most countries, the key to accessing Apple’s Educational Discount programme is being registered with UNiDAYS.
Education Pricing and Student Discounts - Education - Apple
https://www.myunidays.com