
\documentclass{article}
\usepackage{fixltx2e}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{xspace,ifthen}
\usepackage{lmodern}
\usepackage{euscript}
\usepackage[french]{babel}
\pagestyle{empty}
\begin{document}
\newcounter{Lettre}%
\newcommand\LaLettre[1]{%
  \stepcounter{Lettre}
  \(\csname #1\endcsname{\Alph{Lettre}}\)
}%

Avec \verb+\EuScript+: \par
\noindent
\whiledo{\value{Lettre}<26}{%
  \LaLettre{EuScript}%
  \ifthenelse{\value{Lettre}=13}{\par\noindent}{}}

\setcounter{Lettre}{0}%
Avec \verb+\mathcal+: \par
\noindent
\whiledo{\value{Lettre}<26}{%
  \LaLettre{mathcal}%
  \ifthenelse{\value{Lettre}=13}{\par\noindent}{}}
\end{document}
