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In your zone, it should be ok to simply leave them outdoors for winter without any special care. You can, however, add a layer of mulch over top or wrap the container should any unusual wintry weather be expected.

Here is a link with growing information.

Good luck with your assignment.

https://www.gardeningknowhow.com/ornamental/flowers/campanula/growing-campanula-bellflowers.htm

Serbian Bellflower is susceptible to crown rot and mildew under wet conditions.
The top 2 inches of soil should dry out between watering.
Avoid having mulch up to the plant, give it room to help dry out the soil.

https://www.gardeningknowhow.com/ornamental/flowers/campanula/serbian-bellflower-care.htm

Bellflower for the most part are ignored by deer, but when hungry or thirsty enough they'll eat what is available. Thia article may help:
http://www.gardeningknowhow.com/plant-problems/pests/deer/deer-repellents.htm

Invasive it is! The individual stems are all attached to an underground rhizome. Pull the plant and the rhizome lives on. There will also be seed in the soil from past years. Remove the top growth and sun hitting the soil will germinate these seeds. Double whammy. RoundUp will kill it. If you don't want to go that route, remove as much as you can then cover the ground with layers of cardboard. Denying sunlight to new sprouts causes death of the sprout. Expect bellflower to reach past the cardboard and come up. You will have to vigilantly pull these - at least weekly. Eventually you will exhaust the rhizome. To make it look tidier, apply mulch over the cardboard. Since seeds live several years in the soil, removing the cardboard will cause some of these to germinate. It is a several year project to rid your garden of creeping bellflower. good luck

As summer ends pollinated flowers develop seeds or seed capsules. Most members of the bellflower family develop small, hard capsules filled with tiny black or brown seeds. By snipping off the seed capsules and dropping them into a paper bag, you may scatter the seeds in other parts of the garden immediately or save them for spring planting.
https://www.gardeningknowhow.com/ornamental/flowers/campanula/growing-campanula-bellflowers.htm