So, is this what you mean?
\documentclass[12pt,a4paper]{exam}
\usepackage{pgfplots,blindtext}
\begin{document}
\blindtext
\vspace{\baselineskip}
\begingroup
\setlength{\baselineskip}{1mm}
\foreach \i in {1,...,50} {\noindent\rule{\linewidth}{0.2pt}\par}
\endgroup
\vspace{\baselineskip}
\blindtext
\end{document}