Laptop for gaming/school

Lenovo - i7-4510U | 8 GB RAM | 1 TB HDD | NVIDIA GeForce GT 840M [799€]
is the most balanced - middle for both graphics and processor

Dell - i5-4210U | 6 GB RAM | 500 GB HDD | NVIDIA GeForce GT 750M [779€]
Weakest processor but most powerful graphics card

Lenovo - i7-4700MQ | 8 GB RAM | 1 TB HDD | AMD Radeon R7 M265 [779€]
Most powerful processor, by quite a long way, but the weakest graphics card.

The first 2 laptops are both using dual core ultra low voltage processors, so these are much less powerful, but in return will be much thinner, lighter and have significantly better battery life. The 3rd laptop has a regular voltage quad core processor so will be noticabely faster.

However the 3rd of the laptops has the worst graphics card, so purely from a gaming perspective it's the worst choice.

I'd personally go for the first one, it's got the best balance between performance/battery/plenty of RAM/big harddrive. However if gaming is the number 1 priority get the 2nd one. However if you use applications that need a lot of CPU power the 3rd laptop is best.

I'm a big fan of the Lenovo Y series laptops. Looks good to me.

I'd buy the Y50. Better graphics card by quite a long way, better screen. It's got a dual core CPU, but do you do anything that requires a quad core CPU?