Saber
The cross-platform open-source app built for handwriting
.... This allows you to write with white ink on a black background, which is much easier on the eyes in low-light environments like when the teacher turns off the lights in class. Images and PDFs are also inverted, so you can still use a digital printout or a textbook without the fuss. Saber uses a dual-password system to protect your notes from anyone but you, even if they have complete control over the server.