can not implicitly convert type 'string' to 'double'

Here
the fcs_classwork declared as double data type.So you need to convert the value to double

public object coresubjectSecond
       {
           get { return fcs_classwork; }
           set { fcs_classwork = Convert.ToDouble(value);  }
       }

using under code

public object coresubjectSecond
       {
           get { return fcs_classwork; }
           set { fcs_classwork = Convert.ToDouble(value);  }
       }

The Solutions 1 to 3 are good and resolve the above error.

But, it is worthwhile to review the design of the above property.

In the present case as seen from the code given in the question, there seems to be no requirement for having the Type of the property coresubjectSecond as object and the Type of the back end store fcs_classwork as double.

Since, double is a ValueType and object is Reference Type there will be an overhead of boxing and unboxing due to the above design, which may be costly particularly when the property is used excessively.

So, it is better to change the Type of back end store fcs_classwork to object or change the Type of the property coresubjectSecond to double as per the requirement, as a consequence the above error can be avoided as shown below

private double fcs_classwork;

public double coresubjectSecond
{
   get { return fcs_classwork; }
   set { fcs_classwork = value; }
}

Further, as seen from the code the property is being used like a field without any additional code in getter and setter blocks of the property. Hence, if C# 3.0 and above is used, then the auto implemented property can be used in this case which will reduce the code clutter and improve the code readability as shown below

public double coresubjectSecond {get; set;}