The API provides an end point to retrieve information about all submissions to a selected assignment folder (dropbox)
GET /d2l/api/le/(version)/(orgUnitId)/dropbox/folders/(folderId)/submissions/
It does not include a parameter to provide a userId to only retrieve the information for a specific student and so you have to retrieve all, loop through them until you find the student you are looking for. Is there an other API end point to retrieve the submission information for a specific student?