adjusting the top of a chapter

Question

In fact, our university recommends us to use 2.5cm in the top of pages to redact a thesis. for the text is good but when it comes to chapters they are a little bit in the middle which doesn't respect the requirements. I know latex gives good representation and I like it but the requirement are the requirement. So here is the code:

\documentclass[12pt]{report}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{setspace}
\usepackage{mathptmx}
\usepackage[a4paper,right=20mm,left=25mm,top=25mm, bottom=25mm]{geometry}
\title {title}
\begin{document}
\doublespacing
\maketitle
\tableofcontents
\chapter*{A}
\chapter*{B}
\chapter*{C}
\chapter*{General Introduction} 
Blablabla....

\chapter{I}
\chapter{II}
\chapter{III}
\appendix

\end{document}