A teacher asked me if it was OK to teach order of operations using PEMDAS. My answer...be very cautious and then ask yourself why...
PEMDAS is inherently wrong. It actually teaches students the wrong order of operations. The correct order is:
2. Exponents & Radicals
3. Multiplication & Division
4. Addition & Subtraction
This may seem similar but acknowledging that multiplication and division are inverse operations as are addition and subtraction are vital to true understanding later on. We want them to understand what they are doing - not just follow a "rule."
I could go on but I would rather you hear it from others.