Information technology advancements have facilitated the widespread adoption of online education, providing learners with more flexible and convenient means of accessing educational resources and completing learning tasks. To improve the quality of online education and promote collaborative learning, online assessment is crucial in this transition. However, there are concerns about the transparency of the existing online assessment system. Centralized control by educational authorities restricts user access to the assessment process, resulting in a lack of user trust in assessment outcomes. Moreover, the abundance of online users also raises concerns about the reliability of assessors. In this paper, we present "AssessChain," a decentralized system that integrates hybrid blockchains to make all assessment procedures, including set criteria, feedback, calculation, and moderation, visible and verifiable to users. Besides, AssessChain introduces an assessor selection scheme (ASEL) employing an enhanced density clustering algorithm to ensure the reliability of assessors, avoiding low-quality feedback. We have implemented the system and proved that the performance is reasonable in terms of throughput and latency, and the proposed ASEL can generally identify malicious assessors with more than 95% accuracy. By integrating information technology, the system encourages all educational participants to trust the assessment process and results, which helps promote equity in educational assessment.