Improving Elephant Toothpaste experiment
You might try horseradish as a catalyst, rather than yeast. Use the fresh root, not the prepared sauce, and test a few different plants - some are more effective than others.
Cosmetic supply stores stock 6% (20 volume) $\ce{H2O2}$, but that concentration is more likely to cause skin irritation or bleach fabrics.
The most effective catalyst for decomposing  $\ce{H2O2}$ that I've used is potassium permanganate, $\ce{KMnO4}$, but it has a few liabilities:


It's a strong oxidant and can start a fire when mixed with organic chemicals.
It stains skin and fabric anything from pinkish-purple to black. The stain can be removed with sodium bisulfite, $\ce{NaHSO3}$, sold as an iron-stain remover. That can be a fun demonstration but is not suitable for young students to use.