teacher,
Go to View - in the presentation views group click Slide master - on the
slide master tab in the edit master group, click insert slide master, every
slide master will come with 9 different slide layouts. You can give any
slide master it's own background, or you can give the individual slide
layouts a background whichever you need.
Do give the slide master a background on the slide master tab in the
background group, click background styles, chose between the 12 styles
offered, or click edit background at the bottom and make your own.
Does this help?