When I was a first grader my teacher did a unit on eggs. She somehow got her hands on an incubator and some chicken eggs. She kept them on a short counter in the classroom. For 21 days we kept track of what the inside of the eggs were looking like. We drew pictures and wrote stuff down--it was stapled together in a little booklet. And sure enough on day 21 they hatched (except for one "dud"). It was truly one of the most memorable experiences I had as a child.
It'd require a little prep work, finding a farm and incubator, but SOO worth it.
Here's an activity that my kids loved. It demonstrates how eggs are very strong yet thin enough for the chick (or other bird) to emerge. If you've read my posts on the now-quite-forgotten evolution vs. creation thread, you know that I am a creationist and this is one activity I use to show evidence of a creator by showing how the egg is just the perfect thickness and shape for what it has to do. So, if you have the liberty (ie. at Christian school) to add in "aren't they created well?" all the better.
However, without a philosophical bent, the activity is still a blast. Your comment will probably be, "Wow, they are strong, yet the chick can still get out."
Anyway, get three raw eggs (from the grocery store, these aren't for hatching!) and six plastic bottle caps like you find on bottled water or pop bottles. Also get some paper towels and some cleaner (even hand soap and water is okay) because this is slightly messy.
Arrange three of the bottle caps upside down (so that they look like little cups) on a table in a triangle about 6" or so apart. Put one egg in each cap, the tall way (so it's as tall and narrow as can be, not the short and wide way!). Then put a bottle cap right side up on each egg. The bottle caps don't add any strength, all they do is provide a transition between the flat table and the round eggs (or the round eggs and the books).
Now, the fun part, start stacking big hardcovered books on these three eggs. We always weigh the books as we add them. The last time we did this, we got 72.5 pounds of books stacked on these three eggs before they broke.
Then you can have a fun discussion about the shape of eggs and about birds hatching, etc.
You can also shine a light through a raw egg.
You can also spin both a raw egg and a hard boiled egg. The raw one wobbles and the hard boiled spins evenly. This is due to the sloshing in the raw one!
Another cool thing you can do is put two raw eggs (one fresh and one pretty old -- save one in the frig for a few months if you have time) in a pan of water. The old one, which has lost some of its mass to evaporation (the shells are actually a tad porous) floats while the fresh one is still dense and it floats.
You can also float a raw egg in water. Start by putting the egg in a tall glass of tap water. It should sink. Slowly add lots of salt to the glass (remove the egg so you can stir the water easily) and you'll be able to float the egg because salt water is denser than fresh water.
There's also science (is it long chain polymers??? It's been such a long time) in beaten egg whites (like making a meringue). You might want to do a websearch on this and then beat some whites and discuss why they are different than the unbeaten whites and why you can go from slimy unbeaten whites to foamy beaten whites but you can't go back. Once they are beaten, they stay that way. I honestly forget the science behind it but it's interesting.
A friend of mine just taught her kids about Faberge and the eggs he made. Then, she provided them with wooden eggs from the craft store and paint and sequins and ribbons. They really made some beautiful eggs! This is a great art activity for all ages (and all faiths, since it's an artist study).
I'm glad you teach at a Catholic school so that you can add a nice little comment about how nicely God made eggs. You can also mention to your little ones that people start out as a very different kind of egg and right when it starts growing into a baby, before Mom and Dad even know it's growing, there is enough information about that baby to fill 500,000 printed pages! Eggs (human, chicken, or otherwise) are very complex and wonderfully made.
I don't know if you are concentrating on chicken eggs only, but there is a great book called "Chickens Aren't the Only Ones" by Ruth Heller. It talks about all the different animals/insects that lay eggs. After reading it , I make a "step" book called Seven Sensational Eggs. (idea is from an old Mailbox magazine) It starts: On Sunday, out hatched a____. Next page says: On Monday, out hatched a _______. The kids draw and label a different animal/insect/bug for each day of the week. I have also cut out large egg shapes that the children cut in half with a zigzag line. I attach one side with a brad so the egg can be opened. They decide what animal will be inside the egg. We share them with the class and they try to guess what's in the egg. (I have them give clues)