Try this
SELECT Table1.District, Table1.School, isnull(Table1.Students,0) Students, isnull(Table2.Teachers,0) Teachers,
ROUND(CONVERT(Decimal(10, 2), isnull(Table1.Students,0) / CONVERT(Decimal(10, 2), isnull(Table2.Teachers,0))), 3) AS ratio
FROM (SELECT ntblDistrict_1.txtDistrictName AS District, SUM(tblSummaryStudents.intMale) + SUM(tblSummaryStudents.intFemale) AS Students,
ntblPrimarySchoolList.txtPrimarySchoolName AS School
FROM ntblDistrict AS ntblDistrict_1 INNER JOIN
ntblPrimarySchoolList ON ntblDistrict_1.intDistrictID = ntblPrimarySchoolList.intDistrictID INNER JOIN
tblSchool ON ntblPrimarySchoolList.intPrimaryListID = tblSchool.intPrimaryListID INNER JOIN
tblSummaryStudents ON tblSchool.intSchoolID = tblSummaryStudents.intSchoolID
GROUP BY ntblDistrict_1.txtDistrictName, ntblDistrict_1.intDistrictID, ntblPrimarySchoolList.txtPrimarySchoolName
HAVING (ntblDistrict_1.intDistrictID = 9)) AS Table1 INNER JOIN
(SELECT tblSummaryOthers.intTeachersMale + tblSummaryOthers.intTeachersFemale AS Teachers,
ntblPrimarySchoolList_1.txtPrimarySchoolName AS School
FROM tblSummaryOthers INNER JOIN
tblSchool AS tblSchool_1 ON tblSummaryOthers.intSchoolID = tblSchool_1.intSchoolID INNER JOIN
ntblPrimarySchoolList AS ntblPrimarySchoolList_2 ON tblSchool_1.intPrimaryListID = ntblPrimarySchoolList_2.intPrimaryListID AND
tblSchool_1.intPrimaryListID = ntblPrimarySchoolList_2.intPrimaryListID INNER JOIN
ntblPrimarySchoolList AS ntblPrimarySchoolList_1 ON tblSchool_1.intPrimaryListID = ntblPrimarySchoolList_1.intPrimaryListID AND
tblSchool_1.intPrimaryListID = ntblPrimarySchoolList_1.intPrimaryListID
WHERE (ntblPrimarySchoolList_2.intDistrictID = @intDistrictID)
GROUP BY tblSummaryOthers.intTeachersMale + tblSummaryOthers.intTeachersFemale, ntblPrimarySchoolList_2.intDistrictID,
ntblPrimarySchoolList_1.txtPrimarySchoolName
HAVING (ntblPrimarySchoolList_2.intDistrictID = 9)) AS Table2 ON Table1.School = Table2.School