We consider the reference-based approach for Automatic Short Answer Grading (ASAG) that involves scoring a textual constructed student answer comparing to a teacher-provided reference answer. The reference answer does not cover the variety of student answers as it contains only specific examples of correct answers. Considering other language variants of the reference answer can handle variability in student responses and improve scoring accuracy. Alternative reference answers may be possible, but manually creating them is expensive and time-consuming. In this paper, we consider two issues: First, we need to automatically generate various reference answers that can handle the diversity of student answers. Second, we should provide an accurate grading model that improves sentence similarity computation using multiple reference answers. Therefore, our proposed approach to solve both problems highlights two components. First, we provide a sequence-to-sequence deep learning model that targets generating plausible paraphrased reference answers conditioned on the provided reference answer. Secondly, we propose a supervised grading model based on sentence embedding features. The grading model enriches features to improve accuracy considering multiple reference answers. Experiments are conducted both in Arabic and English. They show that the paraphrase generator produces accurate paraphrases. Using multiple reference answers, the proposed grading model achieves a Root Mean Square Error of 0,6955, a Pearson correlation of 88,92% for the Arabic dataset, an RMSE of 0,7790, and a Pearson correlation of 73,50% for the English dataset. While fine-tuning pre-trained transformers on the English dataset provided state-of-the-art performance (RMSE: 0.7620), our approach yields comparable results. Simple to construct, load, and embed into the Learning Management System question engine with low computational complexity, the proposed approach can be easily integrated into the Learning Management System to support the assessment of short answers.