i would not take minimum recommended specs for laptop as a strict guideline.
kellogg's minimum recommended spec for mac is:
Model: 13” MacBook Pro or MacBook Air
Processor: 2.3 GHz Intel Core i5 with 3MB cache
Memory: 4GB RAM
Hard Drive: At least 256 GB SATA hard disk (5400 RPM)
however, i've been using a 2010 13" macbook air with 1.86 GHz Intel Core 2 Duo and 128GB (SSD) and it has been more than enough (even when running windows on VMware). there are several classmates who still use the old school plastic, white macbooks (discontinued) without an issue. it just might be a little slower booting up and opening programs.
if your computer can boot up in under 30 seconds and can multi task between word, outlook, excel, and a browser without slowing down, you're fine.
most people have a 13". i would not recommend anything smaller. excel work is a pain on small screens. if you want something smaller, get a 13-15" laptop and a tablet. i'm seeing more and more classmates with ipads since all reading material and class slides are posted online. check your schools policy on tablets in the classroom. kellogg doesn't allow laptops in classrooms but allow tablets.
if your buying a new laptop, i highly recommend you spring for an SSD rather than a faster processor. also, everybody uses dropbox so no need for external HHD unless you regularly like to back up your system.