You can download as ZIP from here: https://archive.ics.uci.edu/ml/datasets/Student+Performance
The ZIP contains the data in CSV format. To use in Weka you first have to open the CSV file in a text edtior and do the following:
- Replace all ";" with ","
- Remove the empty last line
- Save the CSV file
Then open Weka, and choose Tools->ArffViewer in the main window. In the ArffViewer choose File->Open and the select the CSV file (make sure to change file type below to CSV). Now the data is loaded into ArffViewer so now save it in Arff format instead. After that you can use the Arff dataset file in Weka, e.g. in the Explorer or Experimenter.