What classes to take to start learning about ARM?
You can take this free online embedded course from The University of Texas at Austin: UT.6.02x Embedded Systems - Shape the World.

It's an introductory class into ARM microcontrollers and the C language. The class uses TM4C123 Tiva LaunchPad board from Texas Instruments which is based on an ARM Cortex-M4 microcontroller and it only costs $13!.

Course description:


  This is a hand-on, learn-by-doing course that shows you how to build
  solutions to real-world problems using embedded systems. Each student
  will purchase a Texas Instruments TM4C123 microcontroller kit and a
  few electronic components. This microcontroller has a state of the art
  ARM Cortex M4 processor. The course uses a bottom-up approach to
  problem-solving building gradually from simple interfacing of switches
  and LEDs to complex concepts like display drivers, digital to analog
  conversion, generation of sound, analog to digital conversion,
  graphics, interrupts, and communication. We will present both general
  principles and practical tips for building circuits and programming
  the microcontroller in the C programming language. You will develop
  debugging skills using oscilloscopes, logic analyzers and software
  instrumentation. Laboratory assignments are first performed in
  simulation, and then you will build and debug your system on the real
  microcontroller. At the conclusion of this course you will be able to
  build your own arcade-style game.