Most important of all, their outlets are found in every single neighborhood, usually near schools. They are not located on the main streets but instead, one has to go backstreets to find this chain.
If someone is strict on eating only halal food then Pizza School can be one of places where you can go. They have a couple of choices such as: Cheese Pizza, Vegetable Pizza, Sweet Potatoes Pizza and Shrimp Pizza - ranging from 5,000 won for a basic cheese pizza and going up to 9,000 won for shrimp pizza. However, their most expensive pizza sells for 12,000 won only. In addition, if you know Korean language then ask the person at the counter to minus meat from the numerous pizza varieties that they offer. In this manner, you may have many choices.
If you are coming to Seoul or are visiting places other than Seoul, look for Pizza School for a halal food choices in Seoul. I'm not saying that it's the best pizza in the country however, it is good for the price.
I wrote about this option so that the people who are new to the city or are living/traveling in budget must have an option.