How to remove duct tape from windows?

Actually the safest and fastest way is to use a hair dryer to heat the tape.

Once the adhesive is warm, it will easily peel off.

Any remaining adhesive can then be easily removed with 90% isopropyl alcohol you can buy at any drug store

WD 40 works WONDERS!

Spray, let sit a few min, then rub off with a clean rag.

Might take a few times to get it all off.

Once clean, wash the window with a spoonful of DAWN to a quart of water. Removed the oil.

Scrape it off and use eucalyptus oil to clean up what's left.

This is why they make scrapers. There are a lot of them out there. Ask any hardware store clerk. 

Slowly pull it off and clean the mass off with aloha!

1.  Use a hair dryer to loosen them. That should help.  

 2.  Use WD40.  3.  Single edged razor blade.  It won't scratch the glass; it takes a gem or quartz to scratch glass.  

use an old razor blade or a glass scraper or spray it with glue remover . or even spray it with dieselene to melt the glue , even a hair drying removes glue .

Buy Un-Do. Drip it on the top of the tape and work your way down.  Goo Off is another similar brand, but I prefer Un-Do.

The school janitor may have a product you can use. 

Next time use painter tape. 

Ask your janitor or custodian. That's their job.

You can go at it either by trying to use a razor to peel off the tape, or using a solvent to dissolve the tape's adhesive and then using a razor to get under it and pull it off.  If you just use a razor, you'll probably have to go over the window a second time with a solvent to remove the residue.  I've had good luck with Goo Gone for removing tape adhesive from windows.