Question

Is it possible to have such a nested UI with JQuery/JTemplates/ASP.NET ?

more or less pseudo-code: The important part is the 2nd foreach loop inside the td.

 <table>
            <thead style="font-weight: bold">
                <tr>
                    <td>{$P.lang['pupilName']}</td>
                    <td>{$P.lang['mail']}</td>                        
                </tr>
            </thead>
            <tbody>
                {#foreach $T.table as record}
                <tr>
                        <td>{$T.record.name}</td>
                        <td>{$T.record.mail}</td>
                        <td> // thats the 3rd column containing a listbox/list with documents
                         {#foreach $T.table1 as doc}
                            <ul>
                                <li>excel sheet 1</li>
                                <li>word document 2</li>
                                <li>pdf document 4</li>
                            </ul>                       
                         {#/for}
                        </td>
                </tr>
                {#/for}
            </tbody>
        </table>

Answer 1

Nested for-each should be possible: see this article that illustrates the example.

Only issue that I see with your code is that I believe that line

{#foreach $T.table1 as doc}

should be something like

{#foreach $T.record.table1 as doc}

i.e. you need to refer to a nested table for each record in parent table.