Online learning in STEM subjects requires an easy way to enter and automatically mark mathematical equations. Existing solutions did not meet our requirements, and therefore we developed Inequality, a new open-source system which works across all major browsers, supports both mouse and touch-based entry, and is usable by high school children and teachers. Inequality has been in use for over 2 years by about 20000 students and nearly 900 teachers as part of the Isaac online learning platform. In this paper we evaluate Inequality as an entry method, assess the flexibility of our approach, and the effect the system has on student behaviour. We prepared 343 questions which could be answered using either Inequality or a traditional method. Looking across over 472000 question attempts, we found that students were equally proficient at answering questions correctly with both entry methods. Moreover, students using Inequality required fewer attempts to arrive at the correct answer 73% of the time. In a detailed analysis of equation construction, we found that Inequality provides significant flexibility in the construction of mathematical expressions, accommodating different working styles. We expected students who first worked on paper before entering their answers would require fewer attempts than those who did not, however this was not the case (p = 0.0109). While our system is clearly usable, a user survey highlighted a number of issues which we have addressed in a subsequent update.