I am not getting all the records for a record type relationship

Could you double check to see if the student record has been synced? Also check to see if for that specific class.id you have more than 10 students.

I think you are trying to relate the record whose source is a view(in Entity backed record), we can't get advanced features options when we are using View a source (features like record relations , record sync's are not possible )

Obviously will be a hit to performance but you can get around it w/ an update() and a second query. 

Took some guesses with the data structure but maybe something like this:

a!localVariables(
  local!class: a!queryRecordType(
    recordType: 'Class',
    fields: { 'Class.relationships.Student' },
    filters: a!queryLogicalExpression(
      operator: "AND",
      filters: {
        a!queryFilter(
          field: 'Class.Id',
          operator: "=",
          value: ri!id
        )
      },
      ignoreFiltersWithEmptyValues: true
    ),
    pagingInfo: a!pagingInfo(startIndex: 1, batchSize: 100)
  ),
  local!student: rule!TMP_queryStudent(classId: fv!item['Class.Id']).data,
  todatasubset(
    a!forEach(
      items: local!class.data,
      expression: a!update(
        fv!item,
        'Class.relationships.Student',
        index(
          local!student,
          wherecontains(
            ri!id,
            tointeger(local!student['Student.ClassId'])
          ),
          null
        )
      )
    )
  )
)