List<List<RecordItem>> combinedList = new List<List<RecordItem>>();

public class RecordItem
    {
        public string fieldName { get; set; }
        public string value { get; set; }
    }

[0] [0]fieldName = "StudentId", value = "S1"
    [1]fieldName = "Maths", value = "90"

[1] [0]fieldName = "StudentId", value = "S1"
    [1]fieldName = "Science", value = "70"

[2] [0]fieldName = "StudentId", value = "S2"
    [1]fieldName = "Maths", value = "60"

[3] [0]fieldName = "StudentId", value = "S2"
    [1]fieldName = "Science", value = "50"

[0] [0]fieldName = "StudentId", value = "S1"
    [1]fieldName = "Maths", value = "90"
    [2]fieldName = "Science", value = "70"

[1] [0]fieldName = "StudentId", value = "S2"
    [1]fieldName = "Science", value = "60"
    [2]fieldName = "Science", value = "50"