On Fri, 18 Aug 2017, Henri wrote:

> On Thu, 2017-08-17 at 15:25 +0200, Fabrice Couvreur wrote:
>> Hello,
>> To get the alignment, I had to create three \vbox. Can we simplify this code a bit ?
>> Thank you
>> Fabrice
>
> Use the optional argument of the new framed you defined.

In addition to that, you can also simplify other things as well. This is 
how I would have done it:

% macros=mkvi
\showframe

\setuplayout
    [header=3\lineheight,
     headerdistance=\lineheight]

\setupbackgrounds
   [header]
   [text]
   [
     frame=off,
     bottomframe=on,
     framecolor=darkgray,
     rulethickness=2pt,
   ]

\defineframed[headerframed]
              [
                frame=off, % For visualization set this to on
                height=fit,
                width=fit,
                location=bottom,
                boffset=\lineheight,
              ]

\starttexdefinition MyHeader #where #day #month #year #title #number #time

     \setupheadertexts
       [{\headerframed[align=middle, foregroundstyle=bold, foregroundcolor=red]
             {#title n\high{o}\,#number}}]

     \setupheadertexts
       [{\headerframed[align=flushleft, foregroundstyle=\ssx]
           {Lycée JANSON DE SAILLY \\ \date[d=#day,m=#month,y=#year]}}]
       [{\headerframed[align=flushright, foregroundstyle=\ssx]
           {#where \\ {#time}}}]

\stoptexdefinition

\MyHeader{Seconde}{17}{08}{2017}{Devoir surveillé}{1}{1h\,30m}

\starttext
\input knuth
\stoptext

An added benefit is that you get correct interlinespace for the left and 
right header.

Aditya