introduce \teckidsgeometry:

when called as \teckidsgeometry it just resets stuff that needs
fixups after geometry changes; when called as \teckidsgeometry[foo]
it calls \geometry{foo} first for you

teckids2jahresbericht.cls

@@ -19,7 +19,7 @@
 % of said person’s immediate fault when using the work as intended.
 
 \NeedsTeXFormat{LaTeX2e}%
-\ProvidesClass{teckids2jahresbericht}[2018/08/13 1.0 Teckids Jahresbericht class]%
+\ProvidesClass{teckids2jahresbericht}[2018/08/21 1.1 Teckids Jahresbericht class]%
 
 \PassOptionsToPackage{paper=a4}{typearea}%
 \RequirePackage{fancyhdr}%
@@ -31,11 +31,9 @@
 \RequirePackage{pgfplots}%
 
 \KOMAoptions{fontsize=9pt}%
-\geometry{paperwidth=154truemm,paperheight=216truemm,includeheadfoot,top=7mm,bottom=5mm,twoside,inner=9mm,outer=7mm,nohead,footskip=7mm}%
-% tell fancyhdr to redo its geometry calculations
-\makeatletter\f@nch@setoffs\makeatother%
-% restore lengths damaged during geometry change
-\setlength{\intextsep}{0pt}%
+
+\teckidsgeometry[paperwidth=154truemm,paperheight=216truemm,includeheadfoot,top=7mm,bottom=5mm,twoside,inner=9mm,outer=7mm,nohead,footskip=7mm]%
+
 % adjust for scaled-down text
 \setlength{\parskip}{2.1mm plus 4mm minus 1mm}%
 

teckids2style.sty

@@ -31,7 +31,7 @@
 % • (em dash) → \dash
 
 \NeedsTeXFormat{LaTeX2e}%
-\ProvidesPackage{teckids2style}[2018/08/12 1.8 Teckids TeX/LaTeX style, second epoch]% Teχ/LᴬTᴇX
+\ProvidesPackage{teckids2style}[2018/08/21 1.9 Teckids TeX/LaTeX style, second epoch]% Teχ/LᴬTᴇX
 
 \RequirePackage{accsupp}%
 \RequirePackage[utf8]{inputenc}%
@@ -48,7 +48,7 @@
 \RequirePackage{multirow}%
 \renewcommand{\multirowsetup}{}%
 \RequirePackage{wrapfig}%
-\setlength{\intextsep}{0pt}%
+\setlength{\intextsep}{0pt}% see also \teckidsgeometry below
 \RequirePackage{intcalc}%
 \RequirePackage[writefile]{listings}%
 \RequirePackage{tocloft}%
@@ -167,6 +167,23 @@
 }%
 \teckidshdr{\headmark}%
 
+% Stuff to do at geometry changes
+\makeatletter%
+\def\teckidsgeometry{\@ifnextchar[\teckidsgeometry@BT\teckidsgeometry@BF}%
+\def\teckidsgeometry@BT[#1]{%
+ \geometry{#1}%
+ \teckidsgeometry@BF%
+}%
+\def\teckidsgeometry@BF{%
+ % restore lengths damaged during geometry change
+ \setlength{\intextsep}{0pt}%
+ % tell fancyhdr to redo its geometry calculations
+ \makeatletter\f@nch@setoffs\makeatother%
+}%
+\makeatother%
+% run it initially
+\teckidsgeometry%
+
 % Hurenkinder und Schusterjungen
 \clubpenalty10000%
 \widowpenalty10000%

teckids2urkunde.cls

@@ -19,18 +19,14 @@
 % of said person’s immediate fault when using the work as intended.
 
 \NeedsTeXFormat{LaTeX2e}%
-\ProvidesClass{teckids2jahresbericht}[2018/08/21 0.1 Teckids Urkunde class]%
+\ProvidesClass{teckids2jahresbericht}[2018/08/21 1.0 Teckids Urkunde class]%
 
 \PassOptionsToPackage{paper=a4}{typearea}%
 \RequirePackage{fancyhdr}%
 \LoadClass{scrartcl}%
 \RequirePackage{teckids2style}%
 
-\geometry{hmargin=2cm,top=0.5cm,showframe}%
-% tell fancyhdr to redo its geometry calculations
-\makeatletter\f@nch@setoffs\makeatother%
-% restore lengths damaged during geometry change
-\setlength{\intextsep}{0pt}%
+\teckidsgeometry[hmargin=2cm,top=0.5cm]%
 
 % run parent style constructor
 \teckidshdr[\the\year{}]{Urkunde}%