DropDownListFor does not write back selectedvalue

I know there are more topics on this subject, but or I have a different problem, or i just do not understand the given solutions. So a revisit for this topic What am I missing here?

this reads the selected value into the selectedview property (and writes it back) So, this works just fine.

VIEW
<div class="form-group">
    <div class="col-md-10">
        @Html.DropDownListFor(model => model.Gender,
                new List<SelectListItem>
                {
                    new SelectListItem { Text = "Male", Value = "m"},
                    new SelectListItem { Text = "Female", Value = "f"},
                }
        , new { htmlAttributes = new { @class = "form-control" } })

        @Html.ValidationMessageFor(model => model.Gender, "", new { @class = "text-danger" })
    </div>
</div>

this reads the selected value into the selectedview property (but does NOT read is back)

    <div class="form-group">
    <div class="col-md-10">
        @Html.DropDownListFor(model => model.SchoolID,
                @* Model.SchoolList *@
                new List<SelectListItem>
                {
                    new SelectListItem { Text = "School 1", Value = "1"},
                    new SelectListItem { Text = "School 2", Value = "2"},
                }
        , new { htmlAttributes = new { @class = "form - control" } })
        @Html.ValidationMessageFor(model => model.SchoolID, "", new { @class = "text-danger" })
    </div>
</div>

model and database setup

MODEL

public string Gender { get; set; }
public string SchoolID { get; set; }


DATABASE TABLE
     [Gender] [varchar](50) NULL,
     [SchoolID] [int] NOT NULL,


DATABASE STORED PROCEDURE
    @Gender varchar(50),
    @SchoolID int

Something about model.SchoolID makes it that the selected value can be set correctly, but it does not return a changed selectedvalue to the model, even though SchoolID and Gender are both defined as string in the model. So for the DropDownListFor they both should be of the same type, right? And if so, then I have no clue why one is working and the other is not... So, some light on this issue will be much appreciated.