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\usetikzlibrary{arrows,shapes,positioning,shadows,trees}

\tikzset{
  basic/.style  = {draw, text width=2cm, drop shadow, font=\sffamily, rectangle},
  root/.style   = {basic, rounded corners=2pt, thin, align=center,
                   fill=green!30},
  level 2/.style = {basic, rounded corners=6pt, thin,align=center, fill=green!60,
                   text width=8em},
  level 3/.style = {basic, thin, align=left, fill=pink!60, text width=10.5em}
}

\begin{tikzpicture}[
  level 1/.style={sibling distance=50mm},
  edge from parent/.style={->,draw},
  >=latex]

% root of the the initial tree, level 1
\node[root] {AHS - Lehrplan}
% The first level, as children of the initial tree
  child {node[level 2] (c1) {Allgemeine}}
  child {node[level 2] (c2) {Genau}}
  child {node[level 2] (c3) {Ungeordnet}};

% The second level, relatively positioned nodes
\begin{scope}[every node/.style={level 3}]
\node [below of = c1, xshift=20pt] (c11) {Allg. Wuensche};
\node [below of = c11] (c12) {Allg. Herausf.};
\node [below of = c12] (c13) {Dieser Text ist zu lange, und darum schaut das ganze bloed aus};


\end{scope}

% lines from each level 1 node to every one of its "children"
\foreach \value in {1,2,3}
  \draw[->] (c1.180) |- (c1\value.west);

\end{tikzpicture}

\documentclass[tikz, border=5pt]{standalone}

\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}

\usetikzlibrary{arrows,shapes,positioning,shadows,trees}

\tikzset{
	basic/.style  = {
		draw, 
		text width=2cm,
		drop shadow,
		font=\sffamily,
		rectangle
	},
	root/.style   = {
		basic,
		rounded corners=2pt,
		thin, 
		align=center,
		text width=2.5cm,% <- eingefügt, damit der Text nicht umbrochen wird
		fill=green!30
	},
	level 2/.style = {
		basic,
		rounded corners=6pt,
		thin,
		align=center,
		fill=green!60,
		text width=8em
	},
	level 3/.style = {
		basic,
		thin,
		align=left,
		fill=pink!60,
		text width=10.5em
	}
}

\begin{document}
\begin{tikzpicture}[
    level 1/.style={sibling distance=50mm},
    edge from parent/.style={->,draw},
    edge from parent fork down,% <- eingefügt
    >=latex
]

% root of the the initial tree, level 1
\node[root] {AHS -- Lehrplan}% <- Bindestrich durch Gedankenstrich ersetzt
% The first level, as children of the initial tree
	child {node[level 2] (c1) {Allgemeine}}
	child {node[level 2] (c2) {Genau}}
	child {node[level 2] (c3) {Ungeordnet}};

% The second level, relatively positioned nodes
\begin{scope}[nodes={level 3}]
\node [below of = c1, xshift=20pt] (c11) {Allg. Wünsche};
\node [below of = c11] (c12) {Allg. Herausf.};
\node [below of = c12, yshift=-15pt] (c13) {Dieser Text ist zu lange, und darum schaut das ganze blöd aus};% <- die Option yshift eingefügt
\end{scope}

% lines from each level 1 node to every one of its "children"
\foreach \value in {1,2,3}
	\draw[->] (c1.180) |- (c1\value.west);
\end{tikzpicture}
\end{document}