You can write in you Schooltas books by using the text tool. The text tool is located in the upper right corner of your bookviewer. Click on the "A|" sign to activate the tool.
Once you've activated the tool you can click anywhere on the page to open the text box. Once the box is active, you can start typing.
To move the text box on the page you can drag the blue navigation button in the upper left corner of the box. If you want to delete the text box, you can press the black "x" in the upper right corner of the text box.
When you're done typing, simply press the "done" button at the top of your screen.