Lab instructors, for both high school and undergraduate college level courses, face issues of constricted time within the lab period and limited student engagement with prelab materials. To address these issues, an online prelab delivery system named LabLessons is developed and tested out in a high school chemistry classroom. The system supplements the laboratory experience by providing visualizations and simulations of concepts to prepare students for the practical experiments. The system requires students to answer prelab questions online, which provides immediate feedback and cuts down on last minute copying of answers that instructors anecdotally reported with paper laboratories. Empirical results demonstrate the effectiveness and improved outcomes for students who have used LabLessons. In addition, the ease of use of the system and better preparedness for the lab is noted by the instructor.