Head to Reporting>Custom Report, click on New Report, then at the base of the page Run Report.
You will automatically get the student's UPN, first name, surname, DOB, Year, Form, Sex and Scholar ID. You can delete any column you don't require by clicking on the dustbin icon. To filter for boys, place a M in the gender column and press return. The total numbers will be at the base of the table. Download this for printing. Repeat this process to select F to get the list of girls.
You can also run a sex report in the Student Custom Report (Reporting>Reports>Students>Custom Report):
In this report you can select Gender and any other area you wish to appear in this report. Filter in the same way as the previous custom report.