[itsCody]

You're not giving him any weapons under OnDialogResponse when they first choose.
Also, this system should be per-player to prevent issues.

PHP Code:

new showclass[MAX_PLAYERS], class_assault[MAX_PLAYERS], class_sniper[MAX_PLAYERS];
public OnPlayerConnect(playerid)
{
    showclass[playerid] = 0;
    class_assault[playerid] = 0;
    class_sniper[playerid] = 0;
    return 1;
}
public OnPlayerSpawn(playerid)
{
    if(showclass[playerid] == 0)
    {
        ShowPlayerDialog(playerid, 2, DIALOG_STYLE_LIST, "Class Selction", "Assault-Rank 1\nSniper-Rank 2", "Choose", "");
    }
    else if(showclass[playerid] == 1 && class_assault[playerid] == 1)
    {
        GivePlayerWeapon(playerid, 24, 100);
        GivePlayerWeapon(playerid, 27, 76);
        GivePlayerWeapon(playerid, 16, 3);
        GivePlayerWeapon(playerid, 31, 300);
    }
    else if(showclass[playerid] == 1 && class_sniper[playerid] == 1)
    {
        GivePlayerWeapon(playerid, 4, 0);
        GivePlayerWeapon(playerid, 23, 60);
        GivePlayerWeapon(playerid, 17, 3);
        GivePlayerWeapon(playerid, 34, 200);
    }
    return 1;
}
public OnDialogResponse(playerid, dialogid, response, listitem, inputtext[])
{
    if(response)
    {
        switch(listitem)
        {
            case 0:
            {
                GivePlayerWeapon(playerid, 24, 100);
                GivePlayerWeapon(playerid, 27, 76);
                GivePlayerWeapon(playerid, 16, 3);
                GivePlayerWeapon(playerid, 31, 300);
                showclass[playerid] = 1;
                class_assault[playerid] = 1;
            }
            case 1:
            {
                GivePlayerWeapon(playerid, 4, 0);
                GivePlayerWeapon(playerid, 23, 60);
                GivePlayerWeapon(playerid, 17, 3);
                GivePlayerWeapon(playerid, 34, 200);
                showclass[playerid] = 1;
                class_sniper[playerid] = 1;
            }
        }
    }
    return 1;
}